Worst case scenario, as expected, is that several Republican Senators and House representatives will reject/object the results. This is more of a protest, but they don't expect that they will be able to change the results. In order for that to happen, both Houses of Congress (Senate and House of Representatives) would have to discuss and agree with each objection. In simpler terms, this would never ever ever happen.
*** Unverified reports that, after so much pressure (to break the law) from Trump, Pence won't attend the event. He'll be replaced by Senator Chuck Grassley.