Seival wrote...
Joker: Hello, Commander!
Shepard: Hello.
Joker: So, how do you like Geth Consensus?
Shepard: Well, it was... strange. Never had such feelings before.
Joker: Was?
Shepard: Heh?
Joker: Are you really sure you actually quitted the Consensus?
Shepard: Of course I'm sure! I just passed through that...
Joker: ...Through what?
Shepard: Err...
Joker: Commander, did you press "Exit" button?
Shepard: "Exit" bu... LEGION! WHERE IS EXIT BUTTON?
Legion: We don't use buttons, Shepard Commander, they are constructional weakness...
Shepard: O_o... N... N... NNN... NOOOOOOOOOOOOOO!
Epilogue: Dear "IT"ers. Please, try to disprove this. And when you will manage to do so, I will use exactly the same arguments to disprove your own theory.
Thanks in advance 
Your assuming Shepard made it off Earth, actually everyone on the planet was harvested easilly and the whole of ME3 takes place in a Reaper AI core.
I'm joking of course, Shepard died at the start of ME2, was taken by the Collectors and he's in the skele-reaper (seriously, no-one actually believed that project Lazerus bull did they?)
Just kidding, it was the Eden Prime beacon that got him.
Not really, FTL travel in Mass Effect is the trick (lowering the mass of an object does not allow it to accelerate beyond the speed of light, that's just silly) and races are auto-harvested when they activate their mass-relay.
Actually the whole Mass Effect universe only exists in virtual form. Shepard, the Reapers and everyone else in the galaxy are being manipulated for the amusement (and enragement) of entities beyond their capabilities to perceive or comprehend.
But really the whole universe is just an illusion created by me so I can experience existence subjectively (also, I am God).
Solipsism is fun! I'd like to thank you all (just me really) for taking part in this, the ultimate thought experiment.