general relativity, quantum gravity shit, loops in spacetime etc and ting are one thing, but standing in a phonebox dialing 1967 into an LED display, hitting a button and going back to steal austin powers's mojo is never going to happen. neither is going back to tip yourself off about the grand national winner or the lottery numbers.
In terms of the new LHC at CERN its equally possible that weird shit could happen with black holes and shit that could destroy the planet, but i woulnd't worry about it. you'd hope they'd get the higgs boson ticked though.
(if anyone wants to read some semi interesting shit about this type of ting then get
MC Hawking's "The Universe in a mofo-ing Nutshell Biatches", its well fundaMENTAL.)