There is such a thing as acting in good faith. When you notice an obvious mistake in your favor (even if it's made by The Evil Faceless Corporation of Malevolency), trying to tell someone about it is not only going to protect your ass if it winds up in court, it's just The Right Thing To Do.
(Of course, if they ignore you or send you in circles, then you have a stronger case that they don't want it back all _that_ bad.)
Hopefully you're joking on this part. No large company will ever act or care about the above quote, so its foolish for an individual to do so.
The theme here is that a (small) win is not worth a life-ruining liability.
Now the poster has the opportunity to work for that company again, which would have probably been burned if they fought to keep the money accidentally paid to them.