There are four sidequest that I know can not be completed after completing other quests. TSF bounties: criminals (Citadel Station/Telos), Ghosts to Rest, Cave Crawling(Dantooine) and Vogga the Hutt (Nar Shaddaa).  <_<   
While I agree that more testing would have resulted in a better game, the game on the whole (to me at least, I can not speak for everyone obviously) is not that bad. 
 
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'Redemption' (Dantooine) as well.   
 
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Meh i completed 'Redemption'. So it is possible, there may be a very specific way of doing it however.