Well, that explains my uneven extrusion issues…
That’ll do it. 
Hopefully you have a spare set of gears already but I suppose one could still limp along with that gear until its replacement is printed. Of course the best place to keep spare parts is in a complete spare 3d printer. I wonder if there would be any benefit in making a double-wide set of gears that can handle twice the torque without breaking. Probably not, since I’ve got a huge number of hours on the very same part with my Prusa-Mendel. (looks worn but the two gears are nicely “familiarized” with each other and still strong) There must have been some kind of “interesting” event.
This was caused by the combination of two factors – slic3r’s output being slightly hollow for gears, and the fact that my hobbed bolt has a wobbly head. The big gear wobbled quite a bit, and put axial forces on the small gear. I’m replacing that hobbed bolt with a hobbed pulley right now, so it will hopefully be less of a problem in the future!
Good luck with the mod.
The other option for keeping a spare extruder around is to have two of them. I really need to get around to doing that. 
Yes, but that spare extruder will keep talking to you, telling you to build a whole printer around it. And when you do, you can print trinkets to give out at Halloween twice as quickly.
Oh, I meant having two extruders on the same printer. Unlikely both will go at the same time, and that way, you have an installed spare!
Don’t tempt me! Being able to print soluble support and experiment with polycarbonate are high on my list. Those e3d hot ends sure look nice…
I had to search for e3d hot ends - They look like very nice units, perhaps a good core piece to build another Mendel with since I seem to have “lost” my existing one to my 15 year old engineer of a son. James, thanks for mentioning those hot ends. Direct drive could eliminate broken gear teeth but as to what is best seems to be a matter of personal preference and what the builder wants to do with the rig.