if you say woe betide the person who does something, you mean that they will be punished or cause trouble for themselves if they do that thing Woe betide anyone who plays Ann's tapes without her permission.
1. Used to express the wish for someone to experience misfortune.
2. Used as a warning or allusion to negative consequences: "Even though my dad was no farmer, he was a crack shot and a very good hunter&.... woe betide the unwary moose that wandered down from Canada"(Howard Frank Mosher).