I believed a lot of rumors about GT and movies and filler since ppl on the internet "backed them up" with screen shots and 10 second clips. These included:
-Chibi Pan's fight in the budoukai was actually a filler flashback of Videl's intensive training by Satan. The french manga was still in the namek saga so I totally believed it since chibi Pan looks a lot like teen Videl.
-Movie 12 was a type of filler story that took place before the 28th budoukai. In it, Chibi Buu in Hell assimilated Cell, causing him to become strong enough to escape the afterlife if he chose to. Goku and Vegeta go to stop him, but get killed in the process. Only after getting killed does Vegeta understand the gravity of the situation and agree to fusion. Gogeta then causes Buu to reincarnate. I believed it because: "pics of the opening where Buu kills every Z villain in Hell before assimilating Cell" (the scenes before and after Gohan killed Freeza), as well as clips of Vegeta+Goku (with halos) fusing into Gogeta and one of the Janemba turning into an Ogre. I assumed the manga hadn't reached the budoukai yet and that's why the Ogre was made instead of Uub.
-Bebi Vegeta was Super Saiyan 4 Vegeta.
-Super Saiyan 4 Goku was Goku as the Legendary Super Saiyan, shown only at the end of GT.
-"Pan's little brother" learns to go SSJ3. I remember thinking to myself "Damn! And I thought the end of Z went Super Saiyan crazy!"
-More time travel crap was supposed to be in GT: Super 17 was actually #21 who was never activated in Gero's lab. Since the lab in Mirai Trunks' timeline was never destroyed, it eventually activated #21, who killed Mirai Trunks, stole the time machine and went back to kill Goku (to avenge Dr. Gero) since there was no Goku to kill in his timeline. Seeing a clip of Super 17 fighting SSJ4 Goku, I believed it as GT was famous for rehashing DB/Z storylines.
-I used to think that Paikuhan was Papoye since Papoye was mentioned in the manga but never shown. To this day I'm surprised that they named Paikuhan what they did. It would have worked out perfectly.