Background actors are just what their name describes, actors that hang out in the background of scenes filling in numbers and taking up space. 99% of the time they're forgotten people who don't really get much attention but hey, it's a fun gig for the actors and could always lead to something more, if that's what you're after.
However now and then viewers are graced with one actor that stands out when they're supposed to be blending in.. But hey that's nothing to be angry about! Most of the time it's for doing something really, really dumb.
You see, actors work really hard in their scenes, but when you're an extra or a background actor you might get stuck doing the same scene or the same actions for hours upon hours of filming time... And that leads to mistakes.