Developer Overkill has added microtransactions to Payday 2, after stating, on multiple occasions, that it wouldn't. They're part of the Black Market update, 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 "We've made it clear that Payday 2 will have no microtransactions whatsoever (shame on you if you thought otherwise!)"
The following month, 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 entitled "Fuck you overkill", and here's the Steam page, which is in the process of being drowned in negative reviews.