Oh the days
I remember a incident where on for a a couple of months every Windows 95 computer in the building (approx) 1500 would blue screen simultaneously on a Friday morning. Pretty impressive to watch!
We investigated everything and couldn't find a thing, We even looked at hiring a forensics company but they wanted £300k just to start the investigation.
It got to a point where IT staff would arrange doctors appointments, boiler repairs, holiday, etc just to avoid the Friday morning tirade of angry users.
I finally tracked it down to some developers who were using Novell IPX drivers on DOS based machines, they had written some low level data transfer tools and decided that they would start using some "unused' data fields in the IPX header for their own error messages, and guess when they would run these, Yep Friday morning!
I asked them if they had never noticed that every time they ran these tools that all the workstations crashed, they said they had, but they didn't think is was anything that they were doing.
"Please come outside the IT management team want to introduce you to Mr Baseball bat"