The drugs that are most likely to trigger a badtrip are mosh, acid and speed (unconfirmed). The worse badtrips are mostly got on mosh and acid, while the most enjoyable ones often occurs while consuming a fair ammount of good quality hash or weed, especially mixed with alcohol.
When one has inhaled or otherwise consumed marijuana, it is fairly easy to trigger a badtrip, causing one to laugh to tears and start mumbling incoherently and not being able to stop laughing.
The first way is to get in front of the guy and do the following...
man : yo dude, look in my eyes
pothead : *laughs* mkay
*man slowly moves his hands alternatively back and forth his head and the pothead's head, leaving a way clear for vision beside both hands.*
man : you're walking in a forest, and there are trees, and then you start walking faster *speeds up hands* and you run faster and faster and faster and there are trees everywhere and trees and trees *goes on for a few seconds* and then BANG! *man gives a little hit of the palm in the forehead of the pothead* you run in a tree.
The second way is more of the same, but instead the trees are running at the pothead.
The third way is still more of the same, but the man has to place himself on the side of the pothead, mimic waves with his hands, and replace the trees with waves and the forest with a boat and the running part with a riding boat part.
the fourth way involves using many persons. One of them tells the pothead "I'm placeing a magic box all around you, you can't see it but it filters all the sounds." Then everyone shall move their mouths as they would if they said "You can't hear me, but i'm going to fuck you in the ass", but without making a single noise. then they have to continue saying senseless things without producing sounds.
The fifth way involves two mans to make the pothead get on his knees between them, then one must say "1-2-3-4-5-6-7-8-9-10-1-2-3.." as the others says "A-B-C-D-E-F..." non-stop, repeating themselves. For this one, I recommend holding your victim, else he'll just collapse on the floor laughing.
The sixth way is more of the same, but both mans have to sing a different song in each ear.
Plenty of tricks like these exist, and there are for every drug.