User blog comment:Brianbreed/The Biggest Single Plot Hole in Mass Effect 2/@comment-1221773-20110910031721

I'm certain it's explained in the game that Mordin's Seeker sample was obtained by Cerberus for him. I don't think we know where Cerberus got the sample, but that by itself isn't particularly important to our understanding of the story, and thus not a plot hole. Cerberus could have gotten it from Freedom's Progress or another human colony that was attacked by the Collectors but not specifically mentioned. Stuff happens in the background. Not a big deal.

And I'm not understanding why where the Normandy jumps to to escape the Collector Base explosion is important. It probably jumps to a random location in space to escape the immediate vicinity of the blast, then returns and takes the Tartarus Debris Field Relay (what I'll call the nameless mass relay that the Omega 4 Relay connects to) back to the Omega 4 Relay in the Sahrabarik system. Again, not a big deal.