The biggest issue I have with everything being a simulation theory is that logic would suggest that we wouldn't be able to contemplate such a thing in the first place. Any entity that was advanced enough to create this so-called simulation would almost certainly have built-in detection and mitigation systems to deter and stop that kind of thinking.

Humans just can't, for the life of them, accept that this thing we call reality exists. Someone, somewhere, is always searching for a reason why we exist at all instead of just being content and existing. Let's say that theoretically we are living in a simulation; we'll ignore the obvious massive amounts of power and resources needed to run the world as we know it and forego all the logistics that go into that.

The only real way to definitively prove it was a simulation would be to provoke a runtime error or a mesh error. However, this would be easier said than done; we'd have to find something so incredibly resource-taxing in this world that it would break the matrix we're living in. I'm talking something on a universe level; remember, any simulation that could create all that we know would almost certainly require unthinkable amounts of RAM to prop up this false existence. If you can accomplish that, then you are onto something.