Where's the Fire? (1987)

I was working at one of Melbourne' large stock-broking firms, implementing the interface between their back-office system and the accounting software we marketed, as well as developing customized reports. One morning, a whistle blew. I looked around, thinking it was perhaps a fire drill, but no-one was moving. Oh well, it can't be important, so I worked on. About 15 minutes later, the whistle sounded again, and again no-one moved. 

I asked the client colleague working near me, what was the whistle about. He explained that some 6 months earlier, soon after the back-office software was first installed, they found they were getting intermittent system crashes. The software supplier was called in, and after investigation, they declared that there was a critical administrative update that conflicted with other normal operations. The solution they provided, was a whistle that the administrator had to blow warning other users that a critical update was about to be performed and that the other users needed to exit any update functions they were performing. On completion of the critical update, the whistle was blown again to signal the "all clear".

As a work-around, their simple "hardware" solution worked and was certainly cheap. I don't know whether they actually provided a software interlock on the critical code section in the next software release or not!