The initially trivial delay where the expected arrival time gets pushed back, and back, and back, and back, and back until it becomes very significant.
E.g.
I look at the platform monitor. My train is expected to arrive at 19:00.
18:55, train now expected to arrive at 19:05.
19:02, train now expected to arrive at 19:07.
19:05, train now expected to arrive at 19:10.
FFS would it be better if I left the station now and just cycled the journey?
19:08, train now expected at 19:15.
19:14, train now expected at 19:20.
Repeat until the overall delay is 25+ minutes, at which point it would have been quicker to cycle the full journey. I can see why this happens but it is a shame the full delay doesn't become apparrent within a few minutes, at least then I could decide if it would be better to use another mode of transport.