I was slowly driving a locomotive manually, removing the rails behind me as I went.
I know it shouldn't be possible, but somehow I accidentally managed to remove the locomotive and I found myself standing on water, unable to move.
After re-placing the locomotive, I couldn't board it until I landfilled the tile I was standing on.

Maybe there's a funny corner case that allowed me to remove the loco? I'm afraid I don't remember if the track was curved or not, or whether I was driving forward or in reverse.
I've not been able to reproduce it.

Found the cause: using the destruction planner doesn't prevent you from removing the locomotive that you're in. This also works on land, but isn't really an issue there.

EDIT: I'm also using Nanobots: Early Bots. I considered the possibility that this is an incompatibility with that mod, but found that regular bots will also remove a locomotive even while you're in it. Seems to just be interplay between BBR and factorio-base then.