Certain skins for the Modern Warfare MP5 in Call of Duty: Warzone has been discovered to give the gun stat boosts, specifically in their damage leading to a lower time-to-kill. While the buffs aren't significant, they are incredibly noticeable given that blueprints are only meant to provide cosmetic improvements to guns.
Sym.gg, a website that compares weapons and shows stats on all things Warzone, recently posted a video on Twitter detailing the comparison. These shadow buffs specifically affect the Banisher and Ultimatum skins.
It is currently unclear whether these unannounced buffs are intentional from Raven Software, the development team. Given that the buffs are incredibly slight and also due to the backlash that true pay-to-win elements being introduced into Warzone would cost, it's unlikely that this is intentional and could just be a bug. Still, skeptics might point to this situation as an example of Warzone attempting to give advantages to players that spend more money, a practice seen in many mobile titles such as Clash of Clans.
Last month, Sym.gg revealed the first example of blueprints seemingly affecting guns, though that time it was in reference to the Grav. However, it's notable that the Plot Twist blueprint did not improve the Grav, but rather nerfed its ADS incredibly slightly. This is more credence to assume that these effects are all unintentional, since its unlikely that Warzone would punish players for spending money on a skin.