Its always on client demonstration day that I find a shitload of bugs. Why do I do this to myself?

Feel ya.
I often start introducing bugs during the end of sprints to get the last stuff done in time. A lot of last-minute refactoring and reckless breaking changes will do that.

Sounds like you could use a bit more testing and code reviewing. (Something I wish I had the time for)

@buried it was indeed my own fault, but largely because I hadn't realised something wasn't working properly. Cue attempting to fix the bug and creating several more in the process...