If you could go back in time and do, or bring, one thing to mess up the timeline what would it be?

I'm dumb, thought it was about stealing something from the past to the present. So these answers do fit the prompt

My first reaction was "bring a T-Rex to the year 2000 and threaten them with it". Ended up pestering AI till it agreed to answer a similar question, here's a summary of those answers:

Advanced Surveillance Technology: Bringing back highly advanced surveillance technology from a more technologically developed era could be a game-changer.

This one makes some sense. You go back to 1999 (or future if possible) and bring equipment that can detect and prove the fact beyond reasonable doubt. Enough to cause the count.

Influential Evidence from the Future: Bringing back concrete evidence from the future, such as detailed records of the government's planned election fraud, could be used to expose and prevent these actions. This could include documents, videos, or other irrefutable proof of planned malpractices in the election.

Now this one goes against the prompt somewhat, but it would be the most effective. Although the butterfly effects from proving time travel may cause new issues...