Replace error with PANIC for callbacks from JIT-compiled code.

doc/ext_ffi_semantics.html

@@ -934,6 +934,22 @@ advisable in general. Do this only if you know the C function, that
 called the callback, copes with the forced stack unwinding and doesn't
 leak resources.
 </p>
+<p>
+One thing that's not allowed, is to let an FFI call into a C&nbsp;function
+get JIT-compiled, which in turn calls a callback, calling into Lua again.
+Usually this attempt is caught by the interpreter first and the
+C&nbsp;function is blacklisted for compilation.
+</p>
+<p>
+However, this heuristic may fail under specific circumstances: e.g. a
+message polling function might not run Lua callbacks right away and the call
+gets JIT-compiled. If it later happens to call back into Lua, you'll get a
+VM PANIC with the message <tt>"bad callback"</tt>. Then you'll need to
+manually turn off JIT-compilation with
+<a href="ext_jit.html#jit_onoff_func"><tt>jit.off()</tt></a> for the
+surrounding Lua function that invokes such a message polling function (or
+similar).
+</p>
 
 <h3 id="callback_resources">Callback resource handling</h3>
 <p>