One of the games I used to play often was OpenTTD, which is an open sourced reimplementation of TTD - Transport Tycoon Deluxe - and TTDPatch, a 3rd party patch adding more new features.
Not long after the OpenTTD 0.6.3 release, I stopped playing. Yesterday, I decided to have a look at it again and see what had changed since then, using a nightly build - which despite their name, are actually relatively stable.
I tried it a little, then deleted it, since I had little interest in rekindling my playing it, and no one else here plays it.
Today, Dad decided he wanted to play it again - he and mum do now and then. Except, the last time they played it, was on a custom build, a nightly with a lot of patches I hand applied because it gave features they used.
Unfortunatly, it didn't work. Patching it had broken something, and I didn't have the source code I used to fix it.
So I downloaded today's nightly, put in some newGRFs for Dad to use, and after several small complaints ("It doesn't do this" "I'm missing that") he had it working again.
Which in turn means I now have to keep a copy around on my computer to keep an eye on things, and so I know how to fix anything he breaks. Or finds broken. Making me put it back on again.
Sod's Law in action at it's finest.