The main thing about having a TV show with flashbacks, flashforwards, or even a slow-moving plot, is you that can't have actors aging rapidly throughout the course of what's supposed to be a certain amount of time—let alone the course of one conversation. Considering this was the conundrum that How I Met Your Mother faced when the show premiered all the way back in 2005, producers had to get creative to ensure that their teenaged actors playing Ted's kids—Lyndsy Fonseca and David Henrie—didn't look mid-twenties when a lengthy conversation with their father was over.
So, smartly, the filmmakers decided to film series finale scenes with the kids all the way back in 2005, and save them for the end of the show.
People has the deets:
Those poor kids. Listening to their dad babble on about God knows what for nine years. They missed college!
[via People]