I'll miss tourists getting on the subway at the airport and start looking at the sticker inside the train showing the stations for a different line than they are in
(Metro mixes the trains it uses on different lines apparently and thus most have stickers for a bunch of different lines, with no indication of which one you are in)
(just to be clear this is completely Madrid's Metro fault)
@kitten The solution is clearly to have only a few metro lines with just one or two junctions so even with completely blank trains it's impossible to mix up the only line you can take.
