Reality is a simulation, what's the biggest indicator we missed?

I actually stole this from a Reddit thread [0] and it figured it may do well here, too.

My personal take: The /notion of concepting/ something like a Boltzmann brain. Or rather, any idea that brings forward notions of simulations outside of observed reality. I can't quite put my finger on it, but somehow having the ability to just /create/ these ideas makes me think of other possibilities.

[0] https://www.reddit.com/r/AskReddit/comments/14m5y1i/if_it_came_out_that_we_were_100_living_in_a/