Unfortunately, the side effect of picking it is that it mess up the lock ^^;; and the key is needed to push the pins back into place.
Yeah, so uhmm... because it's a dead-bolt lock, it turns more than a full 180 degrees, causing the spacer (the thing that adds an extra space to the lock, allowing for different keys to open it) to fall into the keyway (the empty space where the key goes).
See diagram here: http://www.gregmiller.net/locks/mitguide/chapter9.html#11
Not thinking all to clearly, I didn't take the proper countermeasures (pushing the long end of the tension tool up against the inside of the keyway, where the flat side of the key would have been.) And so the bad and terrible happened: at least one spacer dropped into the keyway.
As a result, I ended up having to take the lock off the door, dismantle it, and after much trial and error, figured out that there were:
a) 3 different spacers and 5 total pins
b) somehow, the second and third key pins from the back had gotten mixed up? Somehow? It boggles the mind. The entire plug didn't fit back into the hull if I didn't arrange them that way...* (parts diagram: http://www.gregmiller.net/locks/mitguide/chapter2.html)
c) I was absurdly lucky. I think my key was one where all of the spacers were inside the plug. Had that not been the case, I would have been pretty much screwed. Or rather, I would have had a 1/125th chance of guessing which of the three spacers went in the proper pin. In that case, I would probably have contacted the landlord after I got it working, so I could get him to test his key, and then I'd apologize and fix it if it didn't work. Or uh, panic even more. ::whimpers::
*Each spacer is a different height. Each key pin is a different height as well - at least the ones I took out (the last 3). What are the chances that I somehow messed up the settings, and ensured that my key still works? I think it's pretty low: near zero?
Self-reassuring proof attempt 1: that my key was the one where all the spacers were inside the plug -
The total amount of height the key pins+spacers must match the integral of the empty space above my key and below the sheer line. There can only be one possible number.
Self-reassuring proof attempt 2: Since each spacer is a different height, and each key pin was a different height, and assuming I did drop the last three key pins + spacers at some point in time (I could only remember dropping two, but in my panic and cursing, I might have semi-absently assumed that the third was the third and pushed that back in? Maybe? But there was no other combinations that would have worked...
But yeah, have learned how to take apart a basic dead-bolt and key it to a certain key, as necessary. Chances are, I won't be able to do so for most master-slave keys that use spacers, as I would have no clue which key was the master... and I really wouldn't want to try.
And I will never try to pick the apartment dead-bolt lock unless there's an emergency. :P Never again. Jeeze.