Let's assume that a method for FTL has been found, and for whatever reason, it work. Given our current knowledge of physics, what would be the consequences? For example, under what circumstances would events occur out of order from different reference frames?
If you consider FTL travel in the light of our current physical knowledge the current set hypothetical methods are: wormholes, Kerr black holes, the Alcubierre warp drive. Krashnikov tubes, and tachyons (definitely not so popular these days).
Problems with FTL include causality violation, relativistic mass increase prevents crossing the lightspeed barrier.
You can search the tag for FTL on this SE for other information about FTL travel. You will note FTL questions tend to get closed quickly in this neck of the woods.