One issue which seems to cause some consternation between atheists and theists involves a disagreement over how to spell the word "god" - should it be capitalized or not? Which is correct, god or God? Many atheists frequently spell it with a lowercase 'g' while theists, particularly those who come from a monotheistic religious tradition like Judaism, Christianity, Islam, or Sikhism, always capitalize the 'G'. Who is right?
For theists, the issue can be a sore point because they are sure that it is grammatically incorrect to spell the word as 'god,' thus leading them to wonder if atheists are simply ignorant about good grammar - or, more likely, are deliberately trying to insult them and their beliefs. After all, what could possibly motivate a person to misspell such a simple word - and one used so frequently? It's not like they break grammar rules as a matter of course, so some other psychological purpose must be the cause.
Indeed, it would be rather juvenile to misspell God simply in order to insult theists. If such an atheist had so little respect for another person, why even waste the time writing to them in the first place, much less deliberately trying to hurt them at the same time? While that may actually be the case with some atheists who write the word 'god' with a lowercase 'g,' it isn't the normal reason why atheists spell the word in this manner.
To understand why, we need only observe the fact Christians don't capitalize the 'g' and write about the Gods and Goddesses of the ancient Greeks and Romans. Is that an attempt to insult and denigrate those polytheistic beliefs? Of course not - it's grammatically correct to use a lowercase 'g' and write 'gods and goddesses'.
The reason is that in such cases we are talking about members of a general class or category - specifically, members of a group which gets the label 'gods' because people have, at one time or another, worshipped its members as gods. Any time we are referring to the fact that some being or alleged being is a member of this class, it is grammatically appropriate to use a lowercase 'g' but inappropriate to use an uppercase 'G' - just as it would be inappropriate to talk about Apples or Cats.
The same holds true if we are speaking very generally about Christian, Jewish, Muslim, or Sikh beliefs. It is appropriate to say that Christians believe in a god, that Jews believe in a single god, that Muslims pray every Friday to their god, and that Sikhs worship their god. There is absolutely no reason to capitalize 'god' in any of those sentences.
On the other hand, if we are referring to the specific god-concept that a group worships, then it may be appropriate to use capitalization. We can say that Christians are supposed to follow what their god wants them to do, or we can say that Christians are supposed to follow what God wants them to do. Either works, but we capitalize God in the latter sentence because we are essentially using it as a proper name - just as if we were talking about Apollo, Mercury, or Odin.
Confusion is caused by the fact that Christians don't typically ascribe a personal name to their god - some use Yahweh or Jehovah, but that is pretty rare. The name they use happens to be the same as the general term for the class that being belongs to. It's not unlike a person who has named their cat, Cat. In such a situation, there could be some confusion at times as to when the word should be capitalized and when it shouldn't. The rules themselves may be clear, but their application might not be.
Christians are accustomed to using God because they always reference it in a personal manner - they say that "God has spoken to me," not that "my god has spoken to me." Thus, they and other monotheists might be taken aback at finding people who don't privilege their particular god concept and so reference it in a general manner, just as they do with everyone else's god. It's important to remember in such cases that it is not an insult simply to not be privileged.