Along with dictating what weapons you're allowed to use, Battlefield 4's class system restricts what gadgets you can have at your disposal. This helps to define what the function of a given class is (the Engineer is a good anti-vehicle class, for example) and prevents you from being able to have certain combinations of tools--say, a rocket launcher and an ammo box to provide yourself with additional rockets. Thanks to a new glitch, this is no longer the case.
As documented by YouTube user EvilBeastLord (via BF4 Central), this bug allows you to effectively combine two classes. What this means in effect is that you can gain access to gadgets that one player could otherwise not have, like the aforementioned rocket launcher and ammo box combo (combining the Engineer and Support classes), or a revive kit and anti-tank mines (Assault and Engineer).
The process for doing this is relatively straightforward, though it's also something you're very unlikely to have ever done by mistake. You simply set opposing item slots to have "no gadget" and then hit the L1 and R1 or Left Bumper and Right Bumper as you hit the button to spawn, and that empty gadget slot will be filled by the corresponding slot's gadget from the other class.
It's unclear how long it has been possible to do this, but it reportedly works on all platforms. It's been known publicly since as at least as early as July 14, when it was revealed on a Battlefield fan forum.
This glitch isn't necessarily game-breaking in the way others have been; provided you have at least one competent teammate, it's not uncommon to wield a rocket launcher while a teammate's ammo box provides you with additional rockets.
However, this does encourage a lone wolf mentality that goes against the way the game is meant to be played. Players are rewarded in numerous ways for playing alongside teammates, and this gives players more power to go off on their own. As someone who still plays Battlefield 4 and always strives to play in a team-oriented way, that's a real bummer. Hopefully DICE is able to patch this before long.
Battlefield 4's first several months on the market were plagued with problems, but more serious than glitches and exploits was the fact that the game was effectively unplayable for many players. DICE has since greatly improved the game's stability to the point where problems are far less common and it plays much more smoothly. Still, players' confidence in the series has been shaken, which may in part explain why publisher Electronic Arts was willing to push back the launch of the next game in the series, Battlefield Hardline, until early 2015.