This may sound strange as I've never seen anything like this before. I've been putting new cables on my S-Works road bike and when I went to go tension/readjust the cable for the rear derailleur, I noticed it wasn't shifting. Took me a while to figure it out, but this is what I've found:
I'm using Dura Ace 7801 10 Speed shifters
As we're all aware, you thread the cable through the right body and there's a little stop/indent where the cable head rests and this is what the shifter uses to apply/release tension. Somehow my cable head has slipped through the entire body and is on the side that the cable exits shifter body. Does this make sense? So I thread the cable from the left hand side of the shifter body (when looking at the shifter head on), thread the cable through the body and out the right hand side of the shifter body.
So I should have the cable head on the left hand side of the shifter in the indent, but somehow it's slipped through the entire body and is flopping loose on the right hand side of the shifter body (inside the shifter).
I can't figure out how this happened, but now I need to get this cable out and try and rethread it. Any suggestions on how I can do this? I've tried lining up the indents to push the cable back out the way it came in, but the head is now too big to push back through.
Hopefully my description makes sense. Let me know if I can clear it up any (or make it muddier).
I don't know if I should disassemble the shifter (if that's even possible) or if there's another solution.
I, unfortunately, made this mistake just recently. You probably pushed the cable through before returning the shifter to the "normal" (highest gear) position. I pushed it out by lining it up the way you did, and the pushing a small allen key in from the other side, where the cable exits. Then pushed on the cable end to force it through the gap.
Disassembly would be the final option if all else fails. That sometimes doesn't end well.