15. Conan O'Brien Has A Background Cameo
How I Met Your Mother had some pretty impressive cameos in its time - Britney Spears, Katie Perry, Jennifer Lopez and Jorge Garcia have all popped up to join the gang in MacLaren's for an episode or two (and that's saying nothing of Bryan Cranston or Bob Odenkirk, who still popped up even after breaking big in Breaking Bad).
But odds are most of the audience will have missed one of the bigger cameos; Conan O'Brien. Arriving with none of the fanfare that accompanied other guest stars, O'Brien plays an extra in the background of one scene in Season 7 episode No Pressure. Winning the opportunity to appear as part of a charity auction, O'Brien insisted he be an extra, despite the writers keen to give him a more prominent character.