diff --git a/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntime.kt b/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntime.kt
index 09101a17..ca5c6a91 100644
--- a/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntime.kt
+++ b/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntime.kt
@@ -10,6 +10,13 @@ import io.reactivex.rxjava3.disposables.CompositeDisposable
 import io.reactivex.rxjava3.disposables.FormulaDisposableHelper
 
 object RxJavaRuntime {
+
+    private var defaultErrorHandler: RxJavaRuntimeErrorHandler? = null
+
+    fun setDefaultErrorHandler(errorHandler: RxJavaRuntimeErrorHandler?) {
+        this.defaultErrorHandler = errorHandler
+    }
+
     fun <Input : Any, Output : Any> start(
         input: Observable<Input>,
         formula: IFormula<Input, Output>,
@@ -22,12 +29,18 @@ object RxJavaRuntime {
                 type = formula.type(),
                 local = inspector,
             )
+
+            val onError = { error: Throwable ->
+                val handled = defaultErrorHandler?.onError(error) ?: false
+                if (!handled) emitter.onError(error)
+            }.takeIf { defaultErrorHandler != null } ?: emitter::onError
+
             val runtimeFactory = {
                 FormulaRuntime(
                     threadChecker = threadChecker,
                     formula = formula,
                     onOutput = emitter::onNext,
-                    onError = emitter::onError,
+                    onError = onError,
                     inspector = mergedInspector,
                     isValidationEnabled = isValidationEnabled,
                 )
@@ -45,7 +58,7 @@ object RxJavaRuntime {
                     runtime = runtimeFactory()
                 }
                 runtime.onInput(input)
-            }, emitter::onError))
+            }, onError))
 
             val runnable = Runnable {
                 threadChecker.check("Need to unsubscribe on the main thread.")
@@ -56,4 +69,4 @@ object RxJavaRuntime {
             emitter.setDisposable(disposables)
         }.distinctUntilChanged()
     }
-}
\ No newline at end of file
+}
diff --git a/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntimeErrorHandler.kt b/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntimeErrorHandler.kt
new file mode 100644
index 00000000..aa5fb1ed
--- /dev/null
+++ b/formula-rxjava3/src/main/java/com/instacart/formula/rxjava3/RxJavaRuntimeErrorHandler.kt
@@ -0,0 +1,10 @@
+package com.instacart.formula.rxjava3
+
+interface RxJavaRuntimeErrorHandler {
+    /**
+     * @param error [Throwable] that occurred
+     *
+     * @return true if error was handled, false otherwise
+     */
+    fun onError(error: Throwable): Boolean
+}
diff --git a/formula/src/main/java/com/instacart/formula/Exceptions.kt b/formula/src/main/java/com/instacart/formula/Exceptions.kt
new file mode 100644
index 00000000..347224ee
--- /dev/null
+++ b/formula/src/main/java/com/instacart/formula/Exceptions.kt
@@ -0,0 +1,6 @@
+package com.instacart.formula
+
+/**
+ * Thrown when a child formula is added with a duplicate key.
+ */
+class DuplicateKeyException(override val message: String?) : IllegalStateException()
diff --git a/formula/src/main/java/com/instacart/formula/internal/SingleRequestHolder.kt b/formula/src/main/java/com/instacart/formula/internal/SingleRequestHolder.kt
index 14a336fc..3fc1ea22 100644
--- a/formula/src/main/java/com/instacart/formula/internal/SingleRequestHolder.kt
+++ b/formula/src/main/java/com/instacart/formula/internal/SingleRequestHolder.kt
@@ -1,5 +1,7 @@
 package com.instacart.formula.internal
 
+import com.instacart.formula.DuplicateKeyException
+
 /**
  * Holder tracks when object has already been request and throws an error if requested again. This
  * is used to track duplicate requests for a particular key.
@@ -9,7 +11,7 @@ internal class SingleRequestHolder<T>(val value: T) {
 
     inline fun requestAccess(errorMessage: () -> String): T {
         if (requested) {
-            throw IllegalStateException(errorMessage())
+            throw DuplicateKeyException(errorMessage())
         }
 
         requested = true
diff --git a/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt b/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
index d8f52823..198a9b6f 100644
--- a/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
+++ b/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
@@ -7,6 +7,7 @@ import com.instacart.formula.internal.ClearPluginsRule
 import com.instacart.formula.internal.TestInspector
 import com.instacart.formula.internal.Try
 import com.instacart.formula.rxjava3.RxAction
+import com.instacart.formula.rxjava3.RxJavaRuntime
 import com.instacart.formula.subjects.ChildActionFiresParentEventOnStart
 import com.instacart.formula.subjects.ChildMessageNoParentStateChange
 import com.instacart.formula.subjects.ChildMessageTriggersEventTransitionInParent
@@ -1108,7 +1109,9 @@ class FormulaRuntimeTest(val runtime: TestableRuntime, val name: String) {
         }
 
         val error = result.errorOrNull()?.cause
-        assertThat(error).isInstanceOf(IllegalStateException::class.java)
+        assertThat(error).isInstanceOf(DuplicateKeyException::class.java)
+        val expectedMessage = "There already is a child with same key: FormulaKey(scopeKey=null, type=class com.instacart.formula.subjects.KeyFormula, key=TestKey(id=1)). Override [Formula.key] function."
+        assertThat(error?.message).isEqualTo(expectedMessage)
     }
 
     @Test
diff --git a/formula/src/test/java/com/instacart/formula/error/RxJavaRuntimeErrorHandlerTest.kt b/formula/src/test/java/com/instacart/formula/error/RxJavaRuntimeErrorHandlerTest.kt
new file mode 100644
index 00000000..17ef2681
--- /dev/null
+++ b/formula/src/test/java/com/instacart/formula/error/RxJavaRuntimeErrorHandlerTest.kt
@@ -0,0 +1,95 @@
+package com.instacart.formula.error
+
+import com.google.common.truth.Truth.assertThat
+import com.instacart.formula.Action
+import com.instacart.formula.DuplicateKeyException
+import com.instacart.formula.internal.ClearPluginsRule
+import com.instacart.formula.internal.Try
+import com.instacart.formula.rxjava3.RxJavaRuntime
+import com.instacart.formula.rxjava3.RxJavaRuntimeErrorHandler
+import com.instacart.formula.subjects.DynamicParentFormula
+import com.instacart.formula.subjects.OnlyUpdateFormula
+import com.instacart.formula.subjects.TestKey
+import com.instacart.formula.test.RxJavaTestableRuntime
+import com.instacart.formula.test.TestableRuntime
+import org.junit.After
+import org.junit.Before
+import org.junit.Rule
+import org.junit.Test
+import org.junit.rules.RuleChain
+import org.junit.rules.TestName
+import kotlin.IllegalStateException
+
+class RxJavaRuntimeErrorHandlerTest {
+
+    val runtime: TestableRuntime = RxJavaTestableRuntime
+
+    @get:Rule
+    val rule = RuleChain
+        .outerRule(TestName())
+        .around(ClearPluginsRule())
+        .around(runtime.rule)
+
+    private val errorLogs = mutableListOf<String>()
+
+    private val duplicateKeyErrorHandler = object : RxJavaRuntimeErrorHandler {
+        override fun onError(error: Throwable): Boolean {
+            return when (error) {
+                is DuplicateKeyException -> {
+                    errorLogs.add(error.message.orEmpty())
+                    true
+                }
+
+                else -> {
+                    false
+                }
+            }
+        }
+    }
+
+    @Before
+    fun setUp() {
+        RxJavaRuntime.setDefaultErrorHandler(duplicateKeyErrorHandler)
+    }
+
+    @After
+    fun tearDown() {
+        RxJavaRuntime.setDefaultErrorHandler(null)
+    }
+
+    @Test
+    fun `emitting a generic error throws an exception`() {
+        val result = Try {
+            val formula = OnlyUpdateFormula<Unit> {
+                events(Action.onInit()) {
+                    throw IllegalStateException("crashed")
+                }
+            }
+            runtime.test(formula, Unit)
+        }
+
+        val error = result.errorOrNull()?.cause
+        assertThat(error).isInstanceOf(IllegalStateException::class.java)
+        assertThat(error?.message).isEqualTo("crashed")
+
+        assertThat(errorLogs).isEmpty()
+    }
+
+    @Test
+    fun `adding duplicate child logs an exception`() {
+        val result = Try {
+            val formula = DynamicParentFormula()
+            runtime.test(formula, Unit)
+                .output { addChild(TestKey("1")) }
+                .output { addChild(TestKey("1")) }
+        }
+
+        val error = result.errorOrNull()?.cause
+        assertThat(error).isNull()
+        assertThat(errorLogs).hasSize(1)
+
+        val log = errorLogs.first()
+        val expectedLog = "There already is a child with same key: FormulaKey(scopeKey=null, type=class com.instacart.formula.subjects.KeyFormula, key=TestKey(id=1)). Override [Formula.key] function."
+        assertThat(log).isEqualTo(expectedLog)
+    }
+}