A Macintosh running HyperCard 1.2.5 and System Software version 6.0.4 hangs when HyperCard is the active application in MultiFinder and the user receives an incoming InBox 2.2 memo, triggering Inbox's "audible warning".
The problem persists when the same configuration is used with HyperCard 1.2.2 under the same system software (v6.0.4), so it may be caused by the interaction of InBox and the system software.
Turning off the audible warning in InBox eliminates the problem, and is the only solution to the problem as of November of 1989.