Thank you so much for playing!
Currently the punishment only happens when you mine those wrongly marked tile. When that happens, you will get stunned for 1 turn, so it's a free move for all enemies in the map.
But yeah, that's only relevant when there are enemies in the map. When there's none, there's really no consequences for marking everything.
Initially I wanted to make it cost a "bomb extractor" item to mine a marked tile, which means you'll waste that item and a turn if it turns out to be wrong. But I didn't manage to implement it in the game jam xD