Tim's computer, which I'm using a lot lately on account of the whole iMac-being-dead-as-a-dodo thingy, has been adjusted by him so that the mouse is left-handed. I'm not sure why - Tim, so far as I'm aware, isn't left-handed. But anyway.
The computer I use at work hasn't been so adjusted.
I just happen to have started treating it as if it has. So I press the right-hand mouse button and the results are...not always what I expect.