Zac Hall, with a public service message at 9to5Mac:

For now, the bug is triggered when someone replies to a shared watch face in a thread on Messages in iOS 18. The threaded responses feature allows you to have an inline conversation about a specific message that may have been sent earlier in the chat.

If this happens, Messages will repeatedly crash if the user tries to open the conversation in the app. Sending or responding to conversations from other chats directly in Messages is also not easily possible as the app may repeatedly crash.

Once triggered, the bug affects both users. It appears to require responding to the shared watch face from iOS 18. Replying from iOS 18.1 will not trigger the bug.

However, if the user responds in a thread to the shared watch face, Messages will crash on iOS 18.1 beta, iPadOS 18.1 beta, and macOS 15.1 beta as well.

Yikes, we should be seeing an iOS 18.0.1 update pretty soon that includes this, but it is a nasty bug, though.