Had my headset replaced recently by the LBS but the bike was returned in a not like-for-like condition. On the first ride back on it I couldn't quite work out why I felt so much higher up on the road, balance was slightly off and handlebars were squint. So arrived at my destination and had a look around the bike.
Spotted the problem: the stem was on upside down. So en route to Edinburgh on the train I make the decision to correct it, despite thinking that it could be a fiddly job at the best of times never mind on a wobbly train full of sleepy grumpy sheep. Against my better judgement I get the headset cap off, loosen all the bolts, take off the bars, take off the stem, fork falls out, bolts fall all over the floor. Bike grease all over my hands. Exactly as feared. Nightmare. But 20 minutes later it's all back together. Very satisfying. Although there's always that niggling doubt in my mind that I've probably done something wrong (from previous experiences).