I've attempted duplication of this, but I haven't had any luck so far.
What I did:
I was harvesting pumpkins and filling my animal troughs. My pumpkins are near to the troughs all I have to do is pick a pumpkin, pick it up, click the trough, harvest the next button. No need to move around any. (That was all inconsequential, but feel I need to describe what I was doing and how.) I had picked a pumpkin up onto the pointer out of my backpack. I r-clicked on a trough to drop the pumpkin in. In the walk over (was just long enough to do this), a village mate posted something in chat. I went to reply and clicked on the chat bar to give it focus and started typing right away. It instead dropped the pumpkin to the ground and ended up producing a partial completed local map in my backpack. I had absolutely no parchment, ink, or feather on me at the time. My pack was empty except the pumpkins I was harvesting.) The map was of the current section map (500x500) I was in.
Attempts to reproduce:
I have tried recalling what I was trying to type, but can't. I've tried to perform the same just hitting some random keys as long as I at least get in the (a)dventure->ma(p) sequence somehow. Nothing seems to work. I either end up pulling up another menu option (swim, leave village, etc) or nothing occurs.
Some extra information:
I'm using Ender's client v30.03.11. (I've been lazy... so sue me

If this is something known and been untracable, lock it or just delete it. I couldn't find anything similar. Otherwise, this is all the information I can think of.