While it certainly has fared better than most game shows by having the privilege of years of reruns for fans to record, over 100 episodes are still unable to be seen online - or anywhere, for that matter.
Episodes Currently Missing
Of the 525 episodes of every variation (With the exception of DD2K), only approximately 380 are online/in the circuit (Not including the unaired "Old Timers" special unearthed by co-creator Geoffrey Darby in 2015). Every episode of Family Double Dare made is accounted for, including the [very short lived] FOX Primetime series.
Possibly the most notable chunk of the series missing is the second half of the 1987 season of Super Sloppy Double Dare. Only 23 episodes of the allegedly 40 episode run are available. One of the episodes missing is the infamous "Man in the Boat" episode, where Marc loses it over a plastic man in a boat, which was placed on all of the obstacles. This portion of the episode (as well as a few others) can be seen on the 1988 Kids' Klassics compilation tapes, The Inside Slop and The Messiest Moments.
123 of the 130 episodes (First half: 62; Second half: 61) that were taped for Syndication in 1988 exist on the trading circuit. One episode, Cosmic Sparks vs. Slimey Worms, is missing the second round for some reason.
The masters for the second half of the 1987 Super Sloppy are rumored to have been destroyed by a flood, however this hasn't been confirmed. There's no exact numbers for missing episodes from seasons 1-2, and the 1989 episodes of Super Sloppy.
Another episode that is missing - albeit, one that wasn't fit for broadcast - is the test pilot hosted by Geoffrey Darby. It's nothing like the final product and is more of a prototype than anything else. A small portion can also be seen on The Inside Slop, as well as the Double Dare Reunion 30th Anniversary Special.
UPDATE: A user named Adam Barcan uploaded an episode of the '87 SSDD to YouTube, featuring his team "The Schwarzeneggers", albeit in poor aspect ratio quality, making the count 23.
"The Arm Breaking Incident"
This episode was never given a name.
Aside from "Old Timers", there was at least one other episode of Double Dare that was filmed but never aired. The episode involved a child that had a condition that made his bones more fragile than the average contestant on the show. He lied about it on the application and made it onto the show.
During filming of the episode, the predictable happened - the kid broke his arm while doing one of the obstacles on the show. Marc said that he went down the Sewer Shute and didn't get up for a few seconds (He even thought he died). Nickelodeon, fearing a lawsuit over the incident (especially since his father was a lawyer), gave him the seventh's obstacle prize as compensation (As they only got to obstacle 6), and the case was settled. The episode has not seen the light of day, likely due to this incident.