Reported by Paiy on Discord.

This bug was introduced by allowing Jumpers to be assigned to Swimmers. The attached level and replay show the bug; repeatedly assigning Jumpers to a Swimmer whilst the water overlaps terrain allows the lemming to clip-Jump through the terrain.

This is because lems are first raised by 1px when jumping from water; we need to make sure that no terrain occupies that pixel.

Fixed in Commit 45325d65f.