For me, there is no better example then continuing to play Mass Effect 2 after the final mission. I mean for me at the time the final mission was awesome and huge, beginning with a tender scene in which Commander James Shepard gazes longlingly at the picture of Ashley and ending with a fiery climactic showdown in which the whole team/crew escape the exploding Collector Base and Shepard tells the Illusive man that he was ressurrected to save the galaxy... without the help of Cerberus.
Seriously, it was an amazing final mission and yet, when I decided to keep playing for the sake of getting everything done, I was distraught to find the universe feeling quite empty and there characters all kept mentioning plans to prepare the Normandy for the 'suicide mission'.
Clearly unaware that we had just completed said mission. I dunno, it just ruined the feeling for me.
Plus I made a note of going around and covertly rejecting all the other love interests so that there was no way the game would mess up and think I was pursuing a relationship, so now me and most of the NPC's had fuck all to talk about.
Finally I was given two Cerberus based missions to complete, and while in the first one I gave away valuable Cerberus info to the Alliance (I really hate Cerberus you see) in the second there was no way to complete it without helping Cerberus.
So I get an email from them thanking me, even though I have retired Cerberus, blown up their Collector station, told the Illusive man to go to hell and sent valuable detail to the Council, THEY STILL SAY THEY ARE LUCKY TO HAVE ME
I plan to replay it sometime over my holiday break on a different account, probably going to make all the same decisions save for one:
GET EVERYTHING I COULD EVER WANT DONE, DONE WELL IN ADVANCE OF THE FINAL MISSION!!