When a tunneler decides to dig a wall that is in the process of being fortified, the the imp may finish fortifying the wall, and the tunneler will then successfully breach the wall anyway. It will take some time because he will now dig through a fortified wall instead of dirt, so the effect is easily visible for the player.
Reported on Keeperklan, which also included a link to a youtube video. Reproduced in r1821, but reported to be present since the original game.
Please find a logfile, save file, packet file and map to reproduce attached: breach.zip
Unrelated to issue #721, where tunnelers will now always breach walls.
It existed in original game. I was abusing that bug to get one free visible to me cell while fortifiying T walls - so tunneler break that cell but not able to break others and i can drop my imp in that space.