The explanation is a bit technical, but the result is clear enough. Ken Broomfield writes: "In trying to isolate a problem with Window Juggler, my shareware extension, I've discovered a bug in Mac OS 8.1, in the CopyDeepMask API call. The 680x0 version of this function fails to properly remove the arguments from the stack before returning to the caller, with the result that the stack pointer (A7) will be wrong. The MacOS 8.0 version of this function is coded properly. This bug causes Window Juggler to crash after the CopyDeepMask call returns." The problem can occur on 68040 Macs or on Power Macs running software in emulation mode. Ken has also sent a report of this to Apple.
Update: A reply from Apple Apple's Dale Evans, writing both to Ken and myself, states" "It is true that a system patch to CopyDeepMask has an error, but it was not introduced with Mac OS 8.1. It was actually introduced with Mac OS 7.5.2. It exists on all later systems for PCI Macs; it occurs on all PowerPC based Macs/PowerBooks plus the Quadra 660av and 840av. Other 680x0 based computers should be unaffected."
Further: "QuickTime version 2.5 and 3.0 both patch this trap and correct it. Since Mac OS 8.0 and 8.1 both include QuickTime 2.5, those users should not experience a problem. We plan to correct the bug in the next major release of the Mac OS."