Firstly, apart from the magic resistance bug there isnt any bug in the game that everyone gets 100% of the time. The flying dragons is fine on my game and i suspect its because i started a new game, but anyway all bugs (except the one mentioned) are dependant on something else.
Now the main issue, the quest bugs. Most games follow a set order with quests, or only allow you to do a few different quests at a time. Skyrim is different in that you can have an unlinited amount of active quests. So lets take a few different questlines as its impossible to name them all without geting confused. We have a main quest (call that M), Thieves Guild (T), Brotherhood Guild (

I can start all them at once if i want, just start a few or start all of them and mix and match. So i could do M1 (main quest 1), following by T1, then B1, B2, back to T2, then M2 for example. Thats just one way to progress through the game. On top of that i might only get half way through a quest, before deciding to go do something else and finish it off later. Point i am trying to make is there are an infinate number of ways i can tackle those different questlines, and because some are linked, they have a knock on effect. On top of that there is random exploring, wearing different armour and having different skills which all effect the style of play.
So it is impossible to test ALL ways of playing the game. It might be that a bug in the 2nd main quest might have been caused by me doing a Thieves Guild quest, following by a Dark Brotherhood quest and then clearing out a random cave. That could be the sequence needed to cause the bug. So no matter how many testers you have, you cannot test for every single possible outcome because there are so many
And like i said earlier, because only 1 bug is something every gamer will get (even tho some claim they havent), it can be possible to miss them because they didnt follow the exact sequence for the bug to appear. Its only when you get hundreds of thousands of gamers each playing the game that these things come to light.