This happened to me more then once and it annoys me because I (may) get completely different answers.
One example is http://stackoverflow.com/revisions/1771216/list. Because of the retagging, my first few responses were C#/.NET-specific and I had wanted to ask about exceptions in general.
Another occurrence is this question http://stackoverflow.com/questions/1978000/why-string-isnulloremptystr-and-not-str-isnullorempty. I asked why a .NET class was designed the way it was and someone decided to tag it as C# when VB.NET people could have answered just as well. As far as anyone knows I could have been using VB.
I would like people to stop changing other people's tags incorrectly. I don't know of a solution but I thought I'd mention this for awareness.