When users write up posts in editors like Word there are times that some characters get converted to non-standard characters. These characters dont show when using a standard install of SMF. These characters usually are known as "Smart Quotes". If a text editor is used to write the posts and not a word processor, most of the time this would not be an issue. In order to fix this issue, either the posts need to be edited to not include characters that are not in a standard character set, or the database needs to be converted to UTF-8. You can read here further about UTF-8 and converting to it.