diff --git a/lib/syntax_suggest/core_ext.rb b/lib/syntax_suggest/core_ext.rb
index ffbc922eedf172..3918c45d1edbab 100644
--- a/lib/syntax_suggest/core_ext.rb
+++ b/lib/syntax_suggest/core_ext.rb
@@ -33,8 +33,8 @@ def detailed_message(highlight: true, syntax_suggest: true, **kwargs)
         end
       rescue => e
         if ENV["SYNTAX_SUGGEST_DEBUG"]
-          $stderr.warn(e.message)
-          $stderr.warn(e.backtrace)
+          warn(e.message)
+          warn(e.backtrace)
         end
 
         # Ignore internal errors
diff --git a/process.c b/process.c
index a94b1b4fced775..a31843f9314e81 100644
--- a/process.c
+++ b/process.c
@@ -8124,6 +8124,13 @@ ruby_real_ms_time(void)
  *  +:CLOCK_MONOTONIC+, +:CLOCK_PROCESS_CPUTIME_ID+,
  *  and +:CLOCK_THREAD_CPUTIME_ID+ are optional.
  *
+ *  +:CLOCK_THREAD_CPUTIME_ID+ measures the native thread that reads it, not
+ *  the Ruby thread.  Under the M:N thread scheduler (see +RUBY_MN_THREADS+) a
+ *  Ruby thread can move between native threads, so two reads taken from one
+ *  Ruby thread may come from different native threads, and the later read can
+ *  be smaller than the earlier one.  To measure elapsed time there, use
+ *  +:CLOCK_PROCESS_CPUTIME_ID+ or +:CLOCK_MONOTONIC+.
+ *
  *  Certain emulations are used when the given +clock_id+
  *  is not supported directly:
  *
diff --git a/spec/syntax_suggest/integration/ruby_command_line_spec.rb b/spec/syntax_suggest/integration/ruby_command_line_spec.rb
index 02354ceff00eea..761bd760b5d56a 100644
--- a/spec/syntax_suggest/integration/ruby_command_line_spec.rb
+++ b/spec/syntax_suggest/integration/ruby_command_line_spec.rb
@@ -185,5 +185,34 @@ class Dog
         expect(out).to include("Invalid break")
       end
     end
+
+    it "SYNTAX_SUGGEST_DEBUG reports a rescued internal error instead of masking it" do
+      Dir.mktmpdir do |dir|
+        tmpdir = Pathname(dir)
+
+        # Force `SyntaxSuggest.call` to raise so the rescue in `detailed_message` fires
+        monkeypatch = tmpdir.join("raise_monkeypatch.rb")
+        monkeypatch.write <<~EOM
+          require "syntax_suggest/api"
+
+          module SyntaxSuggest
+            def self.call(*args, **kwargs)
+              raise "boom from monkeypatch"
+            end
+          end
+        EOM
+
+        script = tmpdir.join("script.rb")
+        script.write <<~EOM
+          def lol
+            puts "haha"
+        EOM
+
+        out = `SYNTAX_SUGGEST_DEBUG=1 #{ruby} -I#{lib_dir} -rsyntax_suggest -r#{monkeypatch} #{script} 2>&1`
+
+        expect($?.success?).to be_falsey
+        expect(out).to include("boom from monkeypatch")
+      end
+    end
   end
 end
diff --git a/thread.c b/thread.c
index b034522ea4adcd..ebaf61d963e075 100644
--- a/thread.c
+++ b/thread.c
@@ -5272,6 +5272,7 @@ rb_thread_atfork_internal(rb_thread_t *th, void (*atfork)(rb_thread_t *, const r
     rb_native_mutex_initialize(&th->interrupt_lock);
     rb_native_mutex_initialize(&vm->once_lock);
     rb_native_cond_initialize(&vm->once_cond);
+    rb_jit_cont_init(); // cont.c's jit_cont_lock, likewise
     rb_gc_zombie_objspaces_atfork();
     rb_gc_atfork_global_locks();
     rb_generic_fields_lock_atfork();
diff --git a/tool/lib/core_assertions.rb b/tool/lib/core_assertions.rb
index 8b229a25486c19..002c367e94b8e7 100644
--- a/tool/lib/core_assertions.rb
+++ b/tool/lib/core_assertions.rb
@@ -876,10 +876,15 @@ def assert_all_assertions_foreach(msg = nil, *keys, &block)
       end
       alias all_assertions_foreach assert_all_assertions_foreach
 
-      %w[
+      clocks = %w[
         CLOCK_THREAD_CPUTIME_ID CLOCK_PROCESS_CPUTIME_ID
         CLOCK_MONOTONIC
-      ].find do |c|
+      ]
+      # CLOCK_THREAD_CPUTIME_ID is per native thread, and an M:N thread moves
+      # between native threads, so it can go backwards and yield a negative
+      # elapsed time.  Measure process CPU time there instead.
+      clocks.shift if RUBY_DESCRIPTION.include?("+MN")
+      clocks.find do |c|
         if Process.const_defined?(c)
           [c.to_sym, Process.const_get(c)].find do |clk|
             begin