You can get the "[Player] fell off a ladder" death message if you touch a ladder, swim upwards in water, and die of fall damage

The fell off a ladder death message only checks to see if you last touched a ladder and didn't touch ground since then, so you can swim around for a bit, then die of fall damage, and the death message you get will say "[Player] fell off a ladder" even though you swam upwards since then.

Video example: https://www.youtube.com/watch?v=ZTfKfqk5trs