Y2K
I left a programming job 98 or 99 "Mainly Foxpro" and the next office was occupied by the company COBOL guy. He had been writing, maintaining and upgrading a system for a large steel factors for something like 7 years "IIRC it was upto 11MB in flat text format". I popped in for a visit some point during 2000 and was told he suddenly announced he had a new job and bailed out late 99. Turned out his god knows how many lines of COBOL was not Y2k compliant and he figured the job of fixing it could not be done in time.