I must say that I doubt that any crashes or freezes you experience are due to the hat, because there is nothing special about it.

However, if you experience a crash, please post the stacktrace from the details. If you experience a freeze, please use VisualVM to attach to the client and use it to post a complete set of stacktraces.

Or, if you, by "crash", mean that the Java VM itself crashes rather than gives you an error message, then that does indeed mean that the error is unrelated not only to the hat but also to the rest of the game as such but rather is incidental, and that the cause is to find with either your Java setup or elsewhere in your system configuration. Normally, the Java VM would leave a crash dump in its working directory, so if you want help with debugging it, you can post it on the forums.

For the record, is this on the default client or a custom one?

As it turned out, to the extent that this was the case, it only seems to have been true for custom clients, and is therefore not a bug pertinent to this tracker.