In poker, the saying “tap the glass” refers to telling weaker players at the table about their bad play to vent and educate them in the process.
Don’t ever criticize a player who just beat you with a pathetic hand and a miracle river. For one thing no one cares, for another thing the player may leave the table and give you no chance to get your chips back. That means the fish has not only stolen the bait off your hook, but has also swam away.
Here’s an example of how when you tap the glass at the poker table it can have disastrous consequences.