Now in Dragonball Z...The characters such as Goku makes mistakes...or bad decisions some might say. Here below, are a few mistakes or 'bad decisions' made by the characters.
Mistake(s): Believe it or not, the great Goku has made his fair share of mistakes in the series. The biggest would be allowing Dr. Gero to live. By doing that, the androids were able to take over, and it was because Goku allowed Dr. Gero to live that the androids and Cell were created. Another big mistake would have to be transporting Cell to king kai's planet which exploded but allowed cell to get even more powerful.
Mistake(s): Where do I begin?? Vegeta has made more mistakes that probably anyone. The big one being allowing Cell to become Perfect so that Vegeta can have a challenge. Another would be allowing Dr. Gero to awaken androids 17 and 18. Then of course there was the whole Frieza thing in which he was killed by Frieza but at the time, he was really the only one had a shot of a chance against Frieza.
Mistake(s): Gohan has not made a huge amount of mistakes, but nobody is perfect. During the Cell Games, Gohan was holding in a lot of power and basically toying around with Cell even though he honestly could have killed him from the begginning. By not killing Cell right away, Cell started to grow like a balloon and it forced Goku to transport to King Kai's planet where he died. Another mistake would probably be trying to take on Buu when he first saw him because he just got knocked out badly.
Mistake(s): Future Trunks has only made at the most a couple noticable mistakes. The biggest being fighting Cell in the USSJ form which was a very slow form.
Mistake(s): Krillens number one mistake would probably be not destroying Android 18 when he had the chance. If he blew her up, then Cell would not have become perfect. Then Vegeta would have easily killed him.