A blunder is a grave mistake. In sports or in games, it usually refers to an action or a decision that turns a foreseeable win into an irrecoverable loss.
As Red, I here played N6: FELLS, thinking it were the smart thing to do (another 33 points, balancing the rack by dumping four consonants at once, and throwing five tiles while there were only four left in the bag—intending to end the game quick). Emptying the bag, I stared at my rack: QEIP?U. Then I watched in awe as Blue made his move, O1: DOMINES, 122 points (including the 50 bingo-bonus-point and another 32 points from what were left on my rack).
Better is 15E: FELT, leaving a single tile in the bag (or, if you are quite of a teaser-berserker, you might find 2D: FELLS attractive).
The moral is: do not give your opponent the TWS bingo line at the very end of the game. If necessary, do not even empty the bag—leave at least one tile, especially when some big guy is still around (like the Q here).
Although different in motif, the motive here is similar as before: when leading, sometimes we find that the temptation to end the game quick (along with dumping the unwanted tiles and gaining the biggest possible score) is just too great to resist. For one reason or another, somehow we just forget that the game is not over yet. Not quite.
Since we don’t know what Red had got on her rack (there were still a number of tiles in the bag), we have no idea what choices she had. But while the mistake here might not be as big as the one I made in the first example, this is gross nevertheless. With only seventeen tiles left in the bag and no blanks showed up yet, Red did this. Giving me ADDITION, 131 points.
Moral: When there are only one-fifth tiles left (let alone less) and no blank is down yet, then we have to assume that the opponent has already got them both (and the last thing we want to give an opponent with a pair of blanks is a double-triple—two TWS’s within a single blow).