In Canada, we usually tip from 15% to 20%.   15% if the food/service is as expected and 20% if the food/service was exceptional.

At almost all restaurants, if you find something wrong (food or service) and you let them know, 90% of the time, they will move heaven and earth to correct it - and still take the price of that item off of your bill. 

Similar to what you mentioned about your martini, I was recently at a restaurant and the waiter noticed I wasn't liking one of the appetizers.  I mentioned that it seemed to be very bland.  He took it to the back, the chef came out and agreed that it wasn't prepared properly,  I got a fresh plate of it,  and it was removed from my bill.   I left a 20% tip on my bill because, they went above and beyond.