Goku's mistake was not the tournament but

[Lujin_16]

Pushing the button at the end of the Trunks Arc

[RehBeh]

They didn't have another choice. Had Goku haven't pressed the button, Zamasu would won.
And why is that a mistake?

[omaro34]

Its not a mistake. He was already friends with Present Omni King at that point.

The mistake he made was bringing Future Omni-King to the present, he could have found someone else to be the Present Omni King's friend. More importantly, it would have avoided the Zen Exhibition Match from ever happening.

[Boo Machine]

What else was he supposed to do? If there was ever a time to press the button, that was it.

[Ki Breaker]

Pushing the button at the end of the Trunks Arc

What else was he supposed to do?
That's the best decision he had made

[ChronoTwigger]

There's no mistakes.
Only plot points.
Just wait...

[Kishido]

As much as I hate Goku right now.

That he pushed the button and brougt Future Zeno with him might be the key to get rid of both.

No one van beat Zeno... bar Zeno

[Zillamon51]

None of these are "mistakes."

Pushing the button to get Zeno to stop Zamasu from tearing apart time and space was not a mistake, it was necessary. Him holding a tournament in which defeated universes are destroyed was not a foreseeable consequence.

Bringing Future Zeno back was not a mistake. It fulfilled a promise to Present Zeno. Them holding a tournament in which defeated universes are destroyed was not a foreseeable consequence.

The only possible mistake was asking the Zenos to hold a tournament. Beerus (who knows Zeno better than Goku) warned him not to.

[VegettoEX]

Similar to some of the other threads here being locked, this is a discussion point (what little there is in the first post, quite frankly) already being actively handled in other, similar threads. Please be sure to actually read through other threads and topics before making your own! There is nothing to be gained by having your name attached to a thread's creation, I assure you!