I'm not sure what the penalties are, but I did just get off the phone with my current 401k plan administrator. They will mail me a form to fill out, and I need to send it back to them along with copies of my w-2s from both employers (or final pay stubs) so that they can verify the overcontribution amount. I believe this paperwork has to be submitted on their end by April 1st, and then they will send me a check for the difference. Then I should be all set. Not sure what happens if I were to wait until after April 1st/15th, but I guess I don't have to find out this time.
Edited to add: I do have to fill out 2 1099-R forms - one for this year - because the returned contribution counts as income for 2014, and then for some reason I also have to fill out a 1099-R form for 2015 taxes. Not sure why this shows up in two years.