Operator error, adjustment, loose chain or ? Shimano 105 5700 compact double. Bike has less than 400 miles so it's almost new. Chain was in the middle of the rear cog and small ring front. Shifted to the front big ring and chain dropped to the outside. That has never happened to me. Front derailleur was "professionally" adjusted a few hundred miles ago. Thoughts?? Thanks...

Sounds like the limit screw needs adjusting on the front derailleur.

+1 per FD limit screw...perhaps the new FD cable has finally stretched during this break-in period.

Edit: oops, my bad, yes cable stretch should not cause chain to drop on outside
of big ring.

Cable "stretch" would cause the opposite problem, the fd wouldn't shift far enough to reach the big chainring.

However, +1 on tightening the high limit screw but the OP should make very small adjustments,say 1/4 turn at a time. Riders get into trouble by making big adjustments and then can't return to their starting point.

Thanks for the suggestions. I thought the limit screw had nothing to do with cable stretch...

It doesn't.

Noted and thanks; will tighten 1/4 turn and test. Now I need to figure out which is the high limit screw on my fd; will do some research....

Park Tools and Sheldon Brown are the best free online resources for bike repair/maintenance. Make them your first and second stops in either order.

If I were you I'd read them both thoroughly; you'll learn more than you can imagine.

The high gear limit screw, closest to the pedal.

often marked with a L and H

They're generally in line with the travel of the derailleur, so you can think of it as:

The L screw limits the movement inward, and the H screw limits the movement outward.

Thanks for all the replies/suggestions!

If the front derailleur was accidentally rotated that could also possibly cause over shiftting, so make sure it is aligned properly with the chainwheel, then adjust both limit screws. Typically if the derailleur rotates you would also get rubbing where you did not before.

Thanks everyone did this and worked like a charm today.

I'm thinking that even 1/4 turn might be too much. If it was my bike, I'd try 1/8 turn and, if it happened again, do another 1/8.

Actually did about 1.5/8th of a turn. It only did throw the chain once before this adjustment, but I was noticing that it was not shifting in the front as smoothly. After test riding for only 25 miles today it shifts crisply imo.