If we go into the server's upgrade dance, here's one I took part in - forced and ordered because I smelled a rat, but it was my boss talking, so...
Here goes: there was a 3U server running 2K Advanced Server that was running MS SQL Server 7. It was a production server, but it was late in the evening so no one was using it any more - and anyway it had just been restored, if I remember well.
Well, the beast was using a pair of P3-733 (the server model, coming with their own VRMs on a daughter board) and 512 Mb of RAM. The boss wanted to make the SQL server faster (it was several Gb, accessed simultaneously and all the time by 50 employees) by adding more RAM, and since we had a couple of spare 256 Mb sticks...
I told him I had my doubts: at the time, having 1 Gb of RAM was not something anybody saw very often, and I thought making the attempt without preliminary tests (or even, reading some results on similar configs) was foolish. Boy, was I proven right.
See, 32-bit OSes in the NT family can't address more than 4 Gb of physical RAM - accessing the rest via paging. Since at the time having 4 Gb of RAM was a distant dream, it was more or less understood that anything over 4 Gb would be swap space. Add to it a 512 Mb addressing range dedicated to the OS, and only 3.5 Gb can be accessed by apps. Since it is difficult to take into account without a performance impact, many apps merely expect to encounter at most 2 Gb of RAM - thus do any 'internal' addressing on 31-bit. Now then, add 1 bit for parity (just to do it 'old-school'): 30 bit addressing means 1 Gb of RAM max. Well, if this wasn't the explanation for this exact problem, it must have been really close: as soon as I was done installing the extra RAM and rebooted, I noticed extreme sluggishness once the SQL server process had restarted: it was eating up all CPU resources, making the server unuseable.
The boss wanted me to reinstall the server OS. Knowing that it was a very bad solution to a very strange problem, I decided we'd better call a colleague, who told me 'parity' - I was then sure that all we had to do was remove a RAM stick and be done with it. And we did - with tremendous success.
Not my screw-up, but imagine how smug I felt after my boss had sent me a black look meaning 'what did you do to my server?!' after the 'upgrade', and I was proved right?