I'm literally in my fourth year at university studying software engineering. To suggest that every single bug when fixed often causes another bug is downright misinformation. If that happens often it becomes the pesticide paradox; which means the way the game code is structured, or the QA test methods are flawed.You've never done any sort of programming before have you?
It CAN cause another bug, but it's not often it happens. More often what happens is a risk of regression, which is about a previously resolved issue reappearing due to new changes or interactions.