We spent 18 months ensuring all our systems were Y2K compliant, which a number weren't, particularly recently purchased PCs had dodgy BIOS that needed to be patched, so from a software point of view we were pretty safe.
Nevertheless, it was decided we should power down as much as possible on New Year's Eve, and power it all back up on New Year's Day (which was a Saturday, as I recall), just to make sure. This included every PC, which wasn't as silly as it sounded since the PCs we had at the time had a failure rate of about 10% on power cycling, due to dodgy power supplies (part of the problem with a competitive tender process for PC supply driven by line item accounting).
So on Friday afternoon, a group of us go around telling everyone to shut their computers off and go home, until we get to our Comms guys who refused, on the grounds that all this Y2K work was unnecessary and nothing was going to happen. So we moved on.
Saturday morning, a group of us comes in to power everything up, and make sure it's all working, including all the PCs, so we can replace any that die before people return to work on Tuesday. Thanks to our preparedness everything was working fine, except the phone system. Because the Comms guys who had decided there was no way anything could go wrong, did nothing to update the ancient system the PABX was running on.