Every genre show, providing that it lasts long enough, will sooner or later get around to the gag episode, an episode based around a single gimmick rather than a plot or character development.
SUPERNATURAL's Season 4 (recently finished on ITV) had an episode entitled Monster Movie which was shot in lush black and white with a gothic orchestral score to mimic the old Universal black and white monster movies, just as the title suggests.
The greates exponent of the gag episode was BUFFY THE VAMPIRE SLAYER which had all manner of gags. The most famous of these was Once More With Feeling, the musical episode, but no less impressive was Hush which was mostly silent and Superstar where the show was taken over by a minor character. Buffy's spin off ANGEL also has a notable gag episode in which Angel the vampire is turned into a muppet (no really).
In the STAR TREK universe, the main source of the gag episode was the holodeck. Thanks to this wonderful device VOYAGER was able to visit the Saturday morning serials in Bride of Chaotica, and DEEP SPACE NINE redid Bond in Our Man Bashir.
Unfortunately there is a downside to the gag episode when it is not done well. In the wake of CLOVERFIELD, the 'shot by a reporter' episode has appeared in SANCTUARY and SUPERNATURAL and was reworked in BABYLON 5's View From The Gallery to shockingly poor effect.
The gag episode is a sanity saving device for writers looking to put a new spin on a show to freshen it up for the audience and for themselves. This is all well and good if it serves the plot and characters, but if it's done just for the sake of doing it then it falls flat.