That is the gear you are most likely to drop a chain, but it sounds like your chain is either a few links too long, or the limit screw on your RD is adjusted too far outwards. If you can adjust the limit in towards the center a bit more and still shift to the smallest cog, that would certainly help.

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Thanks Ben, will check those ones at the LBS.

Are you 100% sure the mechanical limit has been set appropriately by the shop and / or yourself?

I get all bikes are different potentially due to chain stay lengths...maybe....but I have ridden ~75,000 miles over the last 5 years and never dropped a chain from the 11.

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it was reviewed by the shop, when you refer to mechanical limit you're reffering to chain lenght? Thanks

The rear derailleur has limit screws that you can adjust.

Right on to the previous poster and to clarify the mechanical limit screws on a Di2 RD are basically the same as the limit screws on any mech RD.

Also of note is the Di2 does overshift ever so slightly. So if the limit is set too tight it can actually make the shift to the 11 more difficult as the derailleur runs into the limit....but that's certainly not your issue. If anything you need to tighten the limit.

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Thanks cmscat50 and Jim, will ask that to be reviewed at the shop.

How is there even room between the 11 and the frame for the chain to get stuck in between them? My low limit RD set screw is backed out a lot (it's basically doing nothing right now) and I haven't seen this issue. Could there be something wrong with your cassette? Do you have the proper spacers in the cassette? I would also say that your chain may be 1-2 links too long.

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depends on the particular frame and wheel combo. some wheels have a little more axle showing beyond the cassette, and some frames might have some space for the chain to wedge in.

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The hi limit screw shouldn't be the culprit. A properly tuned DI2 RD will have the hi limit screw backed out one full turn from the point at which the screw contacts the stop to allow the RD to overshift. The movement of the RD is so controlled by the servo motor that it shouldn't ever move where it's not supposed to go, assuming it has been setup correctly.

My money is on the chain being too long, though there should be plenty of tension on the RD to keep the chain taut. Shifting into the smallest cog when on the small chainring could enable this drop if the chain is too long.

limit screw and chain length need to be checked.

Not sure which model rear mech you have, but you might look at this:

https://si.shimano.com/.../DM-R9150-07-ENG.pdf

Page 148, I believe

This manual refers to it as the "end adjust bolt" you may also hear it called the b-limit screw.

Your chain may indeed be "too long" it may also only be "effectively too long" and this screw needs adjustment. Cut according to the their guidance in the manual the chain on my shimano group is "too long" until that screw is properly adjusted which results in a proper chain tension and spacing between the pulley wheels and cassette.

With a properly functioning rear Di2 mech the limit screws won't do anything, however they still need to be adjusted properly (imho) just in case, a jammed chain can get pretty painful (and do a number on your bike frame).

yes, that was strange to me, previously in other bikes when I have dropped a chain it has been in the front chainrings and never this case where chain get stucked between smallest cog and frame, will check that at the bike shop. Thanks Steve

Thanks txtyree/Bdaghisallo/gentry for the info!