We didn't see anything obviously wrong with the part. One bit of communication with the microprocessor was flaky, so it was probably a weak transistor. The cons

We didn't see anything obviously wrong with the part. One bit of communication with the microprocessor was flaky, so it was probably a weak transistor. The consequence was that the Z80 attempted to look up a command in a lookup table, but the command wasn't there so it ended up looping through all of memory, hanging the floppy drive.