I tabbed them for relegation a couple of months ago, but Crystal Palace has dramatically turned around its Premier League season with notable wins over Chelsea, Arsenal, and most recently this past Sunday, Liverpool. If they can defeat a motivated Tottenham Hotspurs side this Wednesday, it would be an incredible (which is higher than amazing) turnaround, and likely insure that they won't be relegated. Relegation is unlikely at this point; I'm pretty sure two more wins would make it impossible.
Christian Benteke, a former Liverpool player, provided both goals in the upset.
Ultimately, I think the difference has been better defense, as they haven't given up late goals when they've acquired a lead, which they with upsetting frequency earlier in the season. And not giving up late goals to the three top-of-the-table teams they defeated is a definite noteworthy accomplishment.