Anyone who's played an old-school shooter based on id Software's more ancient engines will be familiar with a technique — or cheat, depending on your perspective — called "bunny-hopping" or "strafe-jumping". The idea is that, using certain mouse / keyboard inputs, you can continually increase your speed, outrunning your opponents (and fellow teammates). But why does this feature exists and how does it work? It's video explainer time, folks!
Quake 3 is used to show off the problem, however, as the video mentions, the issue can be exploited in many games based on earlier id Tech-based games, such as the original Counter-Strike.
So, why is bunny-hopping / strafe-jumping a thing? It comes down to the physics code, unsurprisingly. Don't worry, there's no need to be a maths master to understand what's happening.
Basically, the player's intended direction, determined by keyboard and mouse inputs and the current direction, are added to together. This final, three-component vector (x, y and z) is not normalised, so its possible in some cases for these components to have a value greater than one.
I'm sure you can see where this is going: as long as the player's movement continues to add to their velocity, it's possible to go faster and faster.
This is a very basic explanation — for example, I haven't talked about why the player's movement isn't just capped — so check out the much more detailed video. It also includes pictures, which are always helpful.
Strafe-jumping physics explained [YouTube]