Y2K+
I gave my notice at a certain IT department in late October 1999 as I had a new job waiting in January. I wasn't working on anything Y2K, my documentation was up to date and I did a good handover but the (contract) project manager appealed to management to keep me on "just in case" of something. I took December as holiday (as there was a "use it or lose it" policy and that same PM had stopped me using it during the summer, leaving me 5 weeks to burn.
January 4th I was back at the office. I chatted to my colleagues, made some tea, browsed the internet and swivelled my chair left, then right. I repeated that for 4 weeks. Occasionally some managers not from the IT department would ask what I was doing these days and I'd tell them. I didn't sabotage anything except that PM's contract.