Just for security ...
Although not always with a literal dart, authors will sometimes cause their characters to black out for any of a variety of reasons; this allows for the character to be easily kidnapped, the antagonist to make his daring getaway, and generally to advance the plot without using too many panels or giving the readers the opportunity to screw things up. It's not uncommon to switch player control to a new character while the original one is down, or to give the unconscious character a dream sequence of some sort.
Examples[]
Hank gets darted to prevent him from learning the location of the Dead Poet's Society.
Not Red Darts so much as Black Hands, but Alanna is knocked unconscious and kidnapped at the end of the "prologue".
Lance gets drunk and wakes up in a cave in the middle of a forest.
Amnesia Man passes out after having his lungs crushed and wakes up with robotic upgrades.
Alex is knocked out with a dart and robbed.
Samuel becomes unconscious several times, especially when he eats or drinks anything.
Luminus is knocked out by a Robo-Spider so he can be transported to the laboratory (which ultimately doesn't quite work).
Lorrex falls out of his window and goes unconscious at the end of his segment. Cerana is sapped and kidnapped, and faints when the pirate ship she was on is destroyed.
Cirno is knocked out in a flashback which is given immediately before Marisa finds her lying unconscious.
Crystal raver seems to be smothered by a lava flow to enter a Dreamatic Interlude.
Marek Larson is knocked out by his own shield when it ricochets off of a window he attempted to break.
Kyle and his team are all knocked out cold when they crash into an electric fence.
At the end of the first part, Sminly gets red darted when the big bad injects him with a mysterious green liquid.
Trope namer, although the darts cause the victim to experience violent hallucinations referred to as some manner of "madness".
Spiff passes out after having an allergic reaction to a sushi meal, and again after he drinks a mysterious liquid. The author follows it up in both cases with You Are Now This Guy.