Whenever people talk about Quirrell, a question that often comes up is, How could Quirrell have taught for more than one year? Because Hagrid sure makes it sound like Quirrell was the DADA professor, took a sabbatical, and then came back "scared of his own subject." But now we know that the position is cursed, and no one has taught more than a year, so what gives? Maybe Hagrid is just wrong. But the other explanation I've heard (somewhere or other...), which I find more intriguing, is that Quirrell did teach for a year before going to Albania - but that because he returned with Voldemort sharing his soul, the curse didn't recognize him as the same person. So he could teach another year.
If that's the case, then there's a delicious irony in Voldemort's return to vapor form at the end of the year. Probably he didn't care about the curse because he intended to swipe the Stone and be off, and didn't plan to be DADA professor ever again. But either the last few professors have had bad luck or the curse is nastier than that. Lupin, outed as a werewolf, got off lightest. Quirrell died, Lockhart lost his memory and ended up in an institution, Barty Jr. got Kissed, Umbridge got traumatized by centaurs, and Snape has a new tear in his soul and is now one of the Ministry's Most Wanted. Maybe the professors in years past just retired or suddenly decided to change careers, but still, taking the job is a risk. Voldemort knows that better than anyone. And in the end, he did not exit as he had planned: instead of escaping with the Stone, he got his host body fried by The Power He Knows NotTM and had to go back to Albania, in a slightly worse position than he had been for the past ten years because now Dumbledore knew for sure he was out there. If Pettigrew hadn't come along, who knows how long he might have been stuck possessing snakes? He couldn't even try the same trick again, because he'd had his year and he blew it. (Unless he went after a different professor. But maybe they aren't as easy to possess as Quirrell was.) He must have been kicking himself after that. And wondering whether he would have succeeded in getting the Stone if only he had managed to avoid the curse by possessing Professor Vector instead.
With the evidence the wizarding world had after the Third Task, I would probably conclude that Crouch's insane, convicted-Death-Eater, Azkaban-escapee son had broken free of the Imperius Curse, sent up the Dark Mark, Imperiused his father in revenge, stolen Moody's identity, and either confunded Harry or collaborated with him about the "Voldemort's Back" story. Crouch Jr. would want to get Harry for making Voldemort disappear, naturally, and rigging the tournament to get Harry killed spectacularly and publicly would be just the sort of Byzantine revenge plot a crazy Voldemort-worshipper might come up with. Confunding Harry just in case the plot failed so he would at least start acting like a loony would fit right in. Or if they collaborated, maybe Crouch Jr. decided it wasn't Harry's fault since he was a baby and it must have been something James or Lily did, and then he and half-cracked Harry decided to use the fear of Voldemort to destabilize the wizarding world. Possibly for different reasons. Either scenario, though unlikely, sounds more plausible than Voldemort returning after some Dark ceremony conducted by a dead man who had lived as an unregistered rat animagus for thirteen years.
But we know that the true memory can be recovered after it's been altered, so wouldn't the sensible thing to do be to get Harry examined by a St. Mungo's healer to see if he had been confunded or had his memory modified? And to at least try to have his mental health evaluated by a professional? What a lame excuse for an attempted-murder investigation!