I really could not narrow it down, hence i wanted to ask the audience here :p , but to name a few :-
1) DNS propagation
2) FTP / Mail - ASCII & BINARY for FTP issues
3) Cannot access Cpanel
4) Mail box getting locked.
5) Third Party software issues & integration.
ETC ETC...what about others ?
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When a client asks for support and the answer is in your FAQ, you could direct them to the FAQ. Or to any tutorial published on your site. Give them a link directly to the answer. And maybe they will check your FAQ or tutorials the next time they're looking for an answer.
I've noticed tutorials have been very popular lately for answering a customers question and thats always good to have them, but it never hurts to have someone friendly to talk with because when you are talking with your client it goes to show you want to help and them and that you really care about them and thats what its all about.