Fixed in commit bc6f87c. Topic remains open to discuss the fix and whether a different fix would be better.
Old behavior when a Swimmer encounters a wall that it cannot step up (or swim upwards) onto:
- If the wall, going downwards, is 3px or less tall, dive
- If the wall is exactly 4px, and the lemming is not a climber, dive
- If the wall is exactly 4px, and the lemming is a climber, climb the wall if possible, otherwise turn around (note that there is no "dive")
- If the wall is 5px or more, turn around (or climb, if a climber and possible to do so)
New behavior:
- If the wall is exactly 3px, dive
- If the wall is exactly 4px, is climbable, and the lemming is a climber, climb it
- If the wall is exactly 4px, and the lemming is not a climber or could not climb the wall, dive
- If the wall is 5px or more, turn around
This alteration presents the fewest possible breaks to existing content possible - the only situation where a level / replay will break is if a climber-swimmer encounters a wall that he cannot climb, that extends 4px below his current position.
The alternative possibility was to simply keep the "dive up to 4px" behavior even for climbers, and only attempt to climb in cases where diving would not be possible. This is a simpler overall rule, but it poses a slightly higher risk of breaking existing content.