How 'bout this scenario............
Customer gives you all the copy and you set it up and input the graphics and send them your ideas. So, the customer spelled everything correctly and knows he has, and is looking at the layout, composition, type styles, colors, white space and balance of the overall project. He is not on spell check duty since he thinks he has come to a fellow professional. He Okays the whole thing and afterward, someone spelled two words wrong and he is steaming. Who is really at fault ?? He... because you have a disclaimer stating you can make mistakes and he's responsible for them or you for not doing your job ??
As mentioned.... I can understand foreign words, names or oddly spelled words, but doesn't anyone double check your work ?? Don't you have proofreaders or is the same person inputting do their own proofing ?? Seems odd.
Now, the customer spells a word or two wrong and you miss it.... you're going to blame them for quickly overlooking a word.... even if it's an easy one and pass the blame off to them ?? You're the professional, but you can't spell, so it's their fault ??
Correct me if I'm wrong, but isn't this what just about everyone on this site does ?? Spell poorly and have literally no concept of grammar, let alone know how to get it done on their own, so they pass the blame off, since they already know they can't do it ??
Then you want to charge them again for doing business with you ??
Doesn't seem like a way to meet new customers if word gets around about your policy on certain things, let alone keep them as repeats.