Hat in Time was 50% off yesterday and since I didn't have money to spend that day I was gonna wait til later in the sale, but now it's only 10% off today. Guess I'll be holding off on buying it til next time there's a sale :( I don't recall this mechnaic being mentioned anywhere.

Look here https://www.gog.com/forum/general/the_winter_sale_begins_grim_fandango_goes_free_c477e/post257

You got ninja’d ;).Cheers

Yep, same happened to "Pillars of the Earth" and so far no reaction if that was a mistake as well or if the actual price in fact is the mistake.