$ cd test/hotspot/jtreg/testlibrary/ctw
$ make
$ cd dist
$ wget https://repo1.maven.org/maven2/com/uralian/ignition_2.11/0.4.0/ignition_2.11-0.4.0.jar
$ ./ctw.sh ignition_2.11-0.4.0.jar

# Internal Error (/home/shade/trunks/jdk-jdk/src/hotspot/share/oops/method.cpp:807), pid=14848, tid=14851
# assert(k->is_subclass_of(SystemDictionary::Throwable_klass())) failed: invalid exception class
#
# JRE version: OpenJDK Runtime Environment (15.0) (fastdebug build 15-internal+0-adhoc.shade.jdk-jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 15-internal+0-adhoc.shade.jdk-jdk, mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1198081] Method::resolved_checked_exceptions_impl(Method*, Thread*)+0x741

Current thread (0x00007fd414093800): JavaThread "main" [_thread_in_vm, id=13873, stack(0x00007fd41f10e000,0x00007fd41f20f000)]

Stack: [0x00007fd41f10e000,0x00007fd41f20f000], sp=0x00007fd41f20c480, free space=1017k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x1198081] Method::resolved_checked_exceptions_impl(Method*, Thread*)+0x741
V [libjvm.so+0x1395c29] Reflection::new_method(methodHandle const&, bool, Thread*)+0x269
V [libjvm.so+0xddbd8b] get_class_declared_methods_helper(JNIEnv_*, _jclass*, unsigned char, bool, Klass*, Thread*)+0x8db
V [libjvm.so+0xddc1a7] JVM_GetClassDeclaredMethods+0xe7
j java.lang.Class.getDeclaredMethods0(Z)[Ljava/lang/reflect/Method;+0 java.base@15-internal
j java.lang.Class.privateGetDeclaredMethods(Z)[Ljava/lang/reflect/Method;+34 java.base@15-internal
j java.lang.Class.getDeclaredMethods()[Ljava/lang/reflect/Method;+20 java.base@15-internal
j sun.hotspot.tools.ctw.Compiler.compileClass(Ljava/lang/Class;JLjava/util/concurrent/Executor;)V+109

----------------------------

After adding more information to assert:

assert(k->is_subclass_of(SystemDictionary::Throwable_klass())) failed: invalid exception class: com/ignition/ExecutionException for method com.ignition.Step.output(Lcom/ignition/FlowRuntime;)Ljava/lang/Object;

Bytecode disassembly shows com/ignition/ExecutionException is interface (!) and com/ignition/Step.class indeed tries to throw it:

$ javap -c -v -p com/ignition/ExecutionException.class
  Compiled from "ExecutionException.scala"
public interface com.ignition.ExecutionException
  minor version: 0
  major version: 51
  flags: (0x0601) ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT
  this_class: #2 // com/ignition/ExecutionException
  super_class: #4 // java/lang/Object
  interfaces: 0, fields: 0, methods: 0, attributes: 4

$ javap -c -v -p com/ignition/Step.class
  Compiled from "entities.scala"
public interface com.ignition.Step<T extends java.lang.Object, R extends com.ignition.FlowRuntime> extends com.ignition.XmlExport, com.ignition.JsonExport

  public abstract T output(int, R) throws com.ignition.ExecutionException;
    descriptor: (ILcom/ignition/FlowRuntime;)Ljava/lang/Object;
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
    Exceptions:
      throws com.ignition.ExecutionException
    Signature: #79 // (ITR;)TT;

Shouldn't / couldn't bytecode verifier catch this? It certainly does not now, even when forced with JAVA_OPTIONS="-Xverify:all".