Arnold K lifted his skinny fist to heaven: 3 nasty consequences of creating a time paradox.
You now have a Paradox Score (all members involved in temporal shenanigans share the same Paradox Score).
1> You stepped on the wrong butterfly.
“…small thing that could upset balances and knock down a line of small dominoes and then big dominoes and then gigantic dominoes, all down the years across Time…” -Ray Bradbury
Things have...changed. Biological things. The effects of your actions have rippled out through time like a rumor through a crowd, and now lifeforms are starting to show it. Each day that passes, more flora and fauna begin to show signs that they are now longer what they were. Time’s flow over the course of eons has altered, added whole eras to their ancestry, and yet somehow the same individuals walk around now. It seems interesting at first but it can only get worse from here.
Every day your Paradox Score goes up by one. Roll the nearest size die, rounding up (i.e. on the start of the fifth day, roll 1d6). The GM rolls on the local encounter table that many times. All the creatures that are “encountered” get a mutation (count repeated encounters as multiple mutations). Use whatever table you like, Scrap’s is interesting but more for players, I find this one sufficiently visible. Once a particular species has five or more, swap in another critter, or start on common humanoids. If the GM wishes to be more atmospheric, beginning the changes with more normal animals - horses, dogs, cattle - to set the tone that something is wrong.
If unaddressed, convert the game into Gamma World when the Paradox Score reaches thirty.
2> Object loop
“Eh, what’s the worst that can happen?” -Bender Rodriguez
You lost an item of some significance to you in your time shenanigan adventure and now it’s caught in an unstable time loop. Your family Goathammer that you use to shatter the serpentmen’s tesseract; the last arrow you fired at the Chronokhan to halt his ritual; the shirt button your grandfather ripped off you before you accidentally killed him on the way to first meet your grandmother - it seems to be haunting you, impossibly. And there are more of them.
Roll a d20. This is your Chaos Number. Every time you roll it on ANY die (bad luck if you roll a 2) you are reminded of what happened. Your Paradox Score starts at one, and each time you roll your Chaos Number it increases by one. Immediately that many copies of the object tumble out of some bizarre place nearby, often rather inconveniently. These copies can be used as normal - they are in every way the regular object, just at different times.
Anytime your Chaos Number comes up in a roll to use them, a wormhole sucks you, anyone else involved, and all the copies back to the moment which caused the paradox in the first place. Good luck explaining yourself (and your seventy one pairs of heirloom glasses) to yourself.
3> Other timelines.
“Obviously, the time continuum has been disrupted creating this new temporal event sequence, resulting in this alternate reality.” -Dr. Emmet Brown
While you were mucking around with time, so was another you, in another time. That other you is a little different, maybe married someone else, or has a different job. And there was another you beyond that, but from a different country. And another, who looks like Bruce Willis. Somehow they’re here now. You have to kill them.
All encounter tables now have a 50% chance of running into an alterna-clone instead of whatever you would normally (check for clones before rolling on the table). Use Zac S’s time golem d100 table to generate the details for each encounter, but apply only to those PCs involved in the temporal antics. These clones want to replace you in this timeline and must kill you to make it happen. Each PC tracks their Paradox score differently. Every time a PC runs into one of their clones their Score increase by one. Once their score is equal to their Charisma, each additional encounter requires a save vs. Death to make sure the clone doesn’t sneakily replace you in the confusion.
If you fail, you die and get replaced. Play as the clone from now on. This stops other copies of you from appearing.