Developer Overkill has added microtransactions to Payday 2 (opens in new tab), after stating, on multiple occasions, that it wouldn't. They're part of the Black Market update (opens in new tab), which allows players to crack open safes to claim random loot drops. You'll need to microtransact £1.60 of real money, to buy consumable drills that will be able to bust them open.
Many other games feature similar systems, but not many of their developers have previously categorically denied that microtransactions would be in their game. In 2013, producer Almir Listo wrote that (opens in new tab) "We've made it clear that Payday 2 will have no microtransactions whatsoever (shame on you if you thought otherwise!)"
The following month (opens in new tab), in response to worries they might be added in the future, former lead designer David Goldfarb replied that, "No. No. God, I hope not. Never. No."
So yes, Payday 2 now features microtransactions. Those bought safe loot-drops comprise weapon skins that can either be used yourself, or traded/sold on the Steam Workshop.
People, obviously, aren't too happy about this change. Here's a big Reddit thread (opens in new tab) entitled "Fuck you overkill", and here's the Steam page (opens in new tab), which is in the process of being drowned in negative reviews.
The Black Market update is part of Payday 2's ten-day Crimefest event (opens in new tab), which kicked off yesterday. It's currently being overshadowed a wee bit. (Thanks, Eurogamer (opens in new tab).)