If I see more than 2 or more short positions (<=2y) I just bin the CV. Mistakes can happen, but more than 2 is a red flag for me. With the exception of being a real contractor, but this is sometimes difficult to say from CV if it was a real fixed time contract (e.g. quite common in UK), or just tax saving arrangement (common in Poland).
Also at least one long position is a bonus, shows the person will stay given the right circumstances.