I'll tip less than 20% even if it wasn't "egregiously bad" but was still bad. Example... food comes out cold because the server never showed up to grab it from the kitchen. Drinks sit empty on the table for 20 minutes at a time. Food and/or drink order is wrong, despite the server writing it down... any one of those things qualifies as "bad" to me. I know some of these things can be the kitchen's fault, or they're super busy/short staffed, and I'll obviously take that into account, but if my drink is empty for 20 minutes, and you're standing around at the bar talking to the bartender... sorry, you're getting a **** tip. Expect 10-15%.
My worst ever experience was at a Buffalo Wild Wings. This is the only time I've ever actually gone and complained to the manager somewhere, actually. Went with my kids and fiance. We were seated, and sat there 20 minutes before the waitress ever came over to greet us and take drink orders. We ordered drinks and an appetizer at the same time. Twenty minutes later... drinks still haven't come, appetizer still hasn't come. The restaurant wasn't busy. This girl had maybe 3 tables(all in one area), and wasn't taking care of any of them. Finally she appears, and she goes and checks the table next to us, and never even looks our direction. Then we see her go over by the hostess podium, and she is just standing there talking to the hostesses. Another 10 minutes goes by with no drinks, no appetizer, and she's completely MIA again. Nowhere I can see in the restaurant. We can overhear her other couple tables complaining too. So I went and talked to the manager who was bartending. Explained to him that we sat for 20 minutes without her ever coming to greet us and take drink orders. Then when we ordered appetizers and drinks, we sat another 30 minutes, and still haven't received either. So he gets someone to cover the bar, and he makes our drinks for us. He then goes in back and brings out our appetizer... which is cold(I can't believe he actually brought the cold one out to us...) So we refuse it, and send it back, requesting a new one.(Cheese curds are awful cold) As he's leaving with the appetizer, the waitress finally shows up with drinks, sees they're on our table already, and walks away with the tray. She doesn't take our food order. The manager comes back out with the appetizer, and never even offers a discount or comp'd drinks or anything. So at this point... we're here an hour and haven't even ordered our actual entrees, so I told him that we've been here an hour and haven't even gotten to order our food yet as the waitress has just vanished again. He says he'll go find her. Waitress finally comes back and takes our food order. Another twenty to thirty minutes goes by, drinks are empty, waitress is nowhere to be found... no food brought out, manager is back to bartending(and Bdubs does not take long to get their boneless wings made). We got up and left. Never went back to that particular Bdubs. Didn't pay, and didn't tip, obviously. They basically got stiffed for one round of drinks that we actually drank. They had to "eat" the food. Sorry, I'm not staying in that situation, whether I ordered food or not.