Chain falling off inner chain ring no matter how L screw is set Bike is a relatively new Cannondale Carbon Synapse with the Ultegra 6800 groupset (although front chainrings are FSA).  Has about 1000KMs on it.

Bike has been great, but I went out on a ride today and about 40K in or so it started losing the chain off the inner front ring.  Prior to this it was tuned perfectly and then suddenly I couldn't pedal more than 10 rotations without the chain coming off (not shifting at all).

I took out my tools and adjusted the L screw to tighten up the gap and it still happened.  Then I thought I'd just keep out of the lowest rear gear so I could just get home (I was stuck going up a big hill).  Still no luck.  Must have tried 12 times and lost the chain every time within 10 feet of starting up again - making adjustments every time.  By the end of it both the front and rear derailleur were adjusted to be pulling the chain away from the inside brutally - just trying to get home. Eventually I had to give up and call for a pickup. 

The only other thing I noticed was that immediately before this happened the rear cogs were getting funny - it kept wanting to shift down (before the hill - was in a higher gear so in the big ring at the font - didn't lose the chain, but was going all squirrelly in the rear).

I can't see what is doing it.  Chain rings don't have any bent teeth. Seems to only really happen under load. I can tighten up the L screw to the point where it is rubbing the chain and it still will manage to come off if I put any load on the drivetrain. I don't even see how that is possible but it is what happens.

I have no idea what is going on - any advice?
I have had this happen due to a worn rear derailleur spring.  the rear derailleur isn't able to take up the slack fast enough.  also check your chain line, bent rear derailleur hanger, front derailleur cage alignment.  see if you can fit a chain catcher on your bike.