Rotten service deserves no tip (nor a return visit to the resaturant).
I usually tip to the next even dollar over 20% (including tax). I do, however, expect the server to actually look at the plate to see if I got what I ordered.
It is simply amazing how often they screw up the order. I can't remember the last time I got exactly what I ordered. Is it really so complicated to leave the croutons off the salad? Is putting the dressing on the side a complex task?
I wonder how many of these celebrities got crappy service and gave lousy tips for lousy service.
My regular practice when I receive lousy service is to tip 15% and never go back. If the manager isn't smart enough to notice the bad service, there's no point in me telling him.
If you're eating at a busy restaurant, then yes, leaving something off a line-prepared item is a few steps more complex.
Your waiter doesn't make the salad - a salad line does, and your waiter has to specify that you want no croutons and the dressing on the side, instead of letting the salad line prepare the salad the way it appears on the menu (which is, I might add, the way the chef thinks it'll be best).
Then the salad line has to take those extra steps, and THEN your waiter has to remember that you had special orders and verify them.