I remember staying up late to see the tick to over from 999,999,999 to 1 billion, thinking "I'll remember this week my whole life". Little did I realise how 60 hours later the whole world would remember.
The system stored timestamps as a string representing seconds since the epoch, but it assumed it would fit in 9 digits. At 1000000000, it started dropping the last digit, so it went back to Sat 1973-03-03 01:46:40 PST, advancing at 10% of real time. It was fixed fairly quickly.
2 days later...
If only it were year 1001 (binary 9)