While doing an oil change on my WRX.
Never having done anything with a subaru before, couldn't find the oil filter. Hunted around for a while, eventually find it about halfway down the block close up against the firewall, (couldn't really see it because it was in a hard to get to spot, but it was round and oil filter sized) with a strange bracket thing behind it....
Remove the bracket thing, go to unscrew it and it falls off in my hands. Have a look at it, and realise that I've removed the back half of the starter motor...
Go to put it back on, but the centre of the motor had popped forwards of the brushes, so when I put it back on the motor forced the little starter motor gearbox thing forwards, which caused the gears inside it to pop off where they should be and all jam up.
That was fun to fix.
And then it turns out that the oil filter is actually really easy to get to, it screws onto the bottom of the block in front of the sump. I had been too lazy to remove the underbody cover to check there.
In my defense, the back half of the starter motor was pretty much exactly filter sized and shape and I was going by feel as I couldn't see it: