As one or two people may know, I recently posted here about an unfortunate experience getting on the wrong train. From this, I learnt the valuable lesson: if your train leaves early, YOU'RE ON THE WRONG TRAIN.
Well, I've just done it again. And learnt a second valuable lesson: if you have booked seat no. 49 in carriage B, and carriage B doesn't have a seat 49, you're also ON THE WRONG TRAIN.
In both cases, I didn't realise my mistake until the train had got going. Someone has helpfully pointed out that these things always happen in threes, so I eagerly await lesson no. 3.