I hate it when bikes in movies do stupid, unrealistic stuff. The scene in Breaking Away, where Dave drafts the 18-wheeler at 55 mph, yet is riding in his small chainring, comes to mind.
But, the worst example is probably that of the bicycle in Butch Cassidy and The Sundance Kid. Not only does Butch, who has never seen a bicycle before, master not only riding, but he masters riding with Etta Place on the handlebars, the first time he ever gets on a bike. Pretty quick on the uptake for a western outlaw, that Butch...
How about the magic Raleigh track bike in Quicksilver? I would love to have a fixed gear bike which magically gains a freewheel whenever I want to ride down some steps.
Just like movies about musicians, written and directed by the tone-deaf, movies featuring bicycles would certainly benefit from some expert consultants on the set.