With the release of M&M 3rd Edition, we have an issue with having most questions be about 2nd, and a few about 3rd, but no tag to differentiate between the two systems with significant differences. There's also the little played 1st edition of Mutants and masterminds, but right now, all of these games are lumped under the single tag [mutants-and-masterminds]. Do we need more tags to differentiate between these systems, and if so, what should they be? [mutants-and-masterminds-xth] is too long to be a tag.
I'm not certain as to the differences between 1st and 2nd, but between 2nd and 3rd, while many things remain the same, there has been a major shift in the system with many of the names remaining the same. There are now 8 ability scores instead of 6, instead of defense, there's parry and dodge as separate things, the skill list has been simplified and the individual skills broadened, and there are fewer effects. A whole lot of the terminology is the same though, even though it's applied differently, so it would be easy to mistake some 3rd edition questions as 2nd (and vice versa).