Fact: How I met your mother final episodes of the series were shot nearly a decade earlier when the show started. Why!? To keep make it seem like the actors never aged.
To keep the children from aging as the series progressed, all the critical scenes involving the children were filmed during the first and second season. Eventually, the series lasted longer than the team imagined and the footage of the mothers children became scarce in later seasons and episodes.
But the final season where Lyndsy Fonseca and David Henrie were filmed at the start of season two where the mother was finally revealed. Thanks to a strict gag order, the few people allowed in the room at the time never leaked the fate of the show’s titular character.
During How I Met Your Mother Season 4, Alyson Hannigan was pregnant in real life, but the show didn’t want to introduce a new child. All scenes involving her belly were covered with props to avoid having to explain the belly, with the exception of the hot dog eating contest where her real stomach is shown under the context she ate a grotesque number of hot dogs to win the merchandise gifts contest.