I find that it largely depends on the keyboard that I'm using, but I can think of two scenarios when it happens to me.

If I'm typing at a steady pace ( no matter the speed), when a finger doesn't get the tactile feedback it expects, it may hesitate while the next finger strikes the next key on the same pace.

And for short commands like, `ls` or `cat`, I often hit the all the letters at nearly the same time which can swap letters.