Commonly that’s because people test for some results that should not occur but something totally random was never tested. Like yeah if you press jump your character increases it height and moves to the right. It is tested if your position is 0, or your position doesn’t exist or your position is the string “hello”. But what happens if you are running backwards and jump? The programmer and/or tester may not of considered that. Really basic example but you should get the idea
Commonly that’s because people test for some results that should not occur but something totally random was never tested. Like yeah if you press jump your character increases it height and moves to the right. It is tested if your position is 0, or your position doesn’t exist or your position is the string “hello”. But what happens if you are running backwards and jump? The programmer and/or tester may not of considered that. Really basic example but you should get the idea