When at a buffet place, or any other place for that matter, do not take your crying 3 year old into the bathroom and seethe "Stop crying or I'll spank you."
Really, don't. All it does is make your kid cry harder. And then if that 3 year old chokes out, "But I just wanted bread," don't compound things by snapping "I said you couldn't have any...now stop crying or I'll spank you!"
And then if your kid says she has to use the bathroom, don't be a bitch and tell her she can't pee until she stops crying.
Don't tell your kid over and over that you're going to hit her if she doesn't stop crying.
Because if you do all that, the Thumper on the other side of the stall door will say "Oh holy shit," and then she'll mutter something unintelligible under her breath, but it will be enough to make you understand that there is someone else in that room, and that someone else finds your behavior unacceptable.
And no, I don't care what kind of day you had. If you want to be a bitch, be a bitch with someone old enough to take it, mature enough to understand the why of it. But your kid? She doesn't deserve that. She doesn't understand it. All she knows is that she wanted bread, couldn't have it, and when she expressed her upset in the way all 3 year olds do, the one person who should be the caretaker of her emotional safety zone threatened to hit her if she couldn't stop crying.
How very, very special.