For you “grammar enthusiasts” here, what is a commonly committed grammatical error that you don’t mind overlooking?

Sentence fragments. Like these.

That depends.  How much cash do you have on you right now?

I will give the Brits, Canadians and Australians a break on their spelling errors.

I sure ain't no grammar enthusiast as one can tell by much of my various writings even so I try to let most of them slide as it really isn't that important in the long run as we all make mistakes and some people have eyesight issues which make things like, punctuation, hard to see even on; 500% magnification.  Some have the fat finger factor meaning that even with a large keyboard theut fingers keep hitting more than one key at the same time.  And spell checkers ain't much help as the error must be within their range or they says it's perfectly ok. even tho it ain't.
So long as it be readable I don't mind a few grammatical errors;  In fact there are a few peoples out there who wouldn't even notice them as not everyone is a grammar fanatic.  They got better things to do with their life.

None. Nada. Zip-a-dee-doo-dah.

its vs. it's

It's a common error, it can be confusing, and often I think the error is made typing too fast and not out of ignorance. 

I am willing to overlook any error when pointing it out would be unkind, unnecessary, unwanted or cause someone to be less willing to communicate for fear of making a mistake.

Ending sentences with prepositions.
It's not a hill upon which I intend to die.

Totally depends upon the circumstances. It's hard for me to overlook most things when it's a business document, but I accept that everyone has different skills, talents, aptitudes, etc.