If the belt was loose enough it can skip a tooth on shutdown or acceleration, happened to me, pulled it off the trailer at camp and did a hot lap halfway around camp and it shut right off, skipped a tooth on the timing belt, borrowed someone's phone that had service to get the timing marks online, fixed it, then rallied the rest of the weekend!
Now every time I do a timing belt after I get the belt on I go ahead and use a big adjustable wrench around the crank snout to hit the key and turn the crank twice in the right direction to verify the timing marks are lined up then lock down the tensioner and reassemble... I take shortcuts too often, I think this step is worth the extra 2 minutes...