I have always thought that as a general rule, the vav conjunctive prefix is usually a "וְ". But, when the word being "prefixed" either starts with a shva or starts with a "lip-letter" (בומ"פ), the vav changes to a "וּ" (kubutz).
However, I have come across a few of exceptions to the above rule, only one of which I actually remember at the moment. (Other examples are welcome as comments or as part of an answer.)
The exception I can think of right now is Exod. 10:8: "מִי וָמִי הַהֹלְכִים", in which I would think it should be "מִי וּמִי הַהֹלְכִים".
Why is it not "מִי וּמִי"? Is my above rule wrong or missing something, or is this just a particular exception, and if so, why?