If you don't test your NPCs, unforeseeable things could happen:
Haine: Hmm, there are no bug reports on the eA bug tracker, and the code seems good and simple, so I guess I'll just go implement it.
*implements reset NPC*
Haine: So far so good!

Tesla: Haine, there's a ticket about a player who couldn't log in after using the NPC.
Haine: Huh? That's weird, I thought I had added the ... hmm ...
Tesla: There seem to be several players in Prontera dropping lots of valuable items.
Haine: Wha- where'd they get these items?
*looks at item logs*
Haine: The NPC added 100000 Zeny to their characters?! I don-
Talis: Hey, is anyone currently using the item logs? The server lags as hell, and the isp doesn't know anything!
Haine: This shouldn't be beca-
Schrodinger: PRONTERA IS DOWN! And there are lots of MVP spawns in newbie and KS maps, and by lots, I mean like 100!! Did anyone break BBs or is Zidane going crazy with his events?!?
Haine: But I-
Tesla: I was just disconnected while doing an item recovery, can you please stop whatever you're doing?
Haine: It's not my fau-
*Schrodinger logs out*
Talis: The account server displays 9000 logged in accounts, woot. Maybe that's the reason for the lag!
Haine: We didn't have THAT MUCH active playe-
Talis: Our servers can't deal with that many players. Maybe we should rent more machines...
Haine: Schrod, can you check the servers?
Haine: Hey, Schrod, are you there?
Tesla: He's offline.
*Haine calls Schrodinger's cell phone*
[Haine]: Hey, where are you?!
[Schrodinger]: Sorry, my PC suddenly crashed, and I can't get it back to work. Ever since you implemented that NPC I -- oun'-- pr--le-s ---ev--r ...
[Haine]: Huh? I don't understand you, hello? You still there?
Tesla: Can't access the forums anymore!
Zidane: Hey, there's something wrong with the forums!
Kirek: Server connection is dead. Test server not working, neither is the patcher nor any client.
Lucifiel: Heey~ several players who added me in MSN ask what's wrong with the forums and the server, because they apparently can't get on. Can you maybe help them?
RSX-0806: Is anyone watching the news?! They say, there was another oil disaster by BP!!!
Talis: Haine, you there? We have a huge problem, the ISP called and said our server exploded, what did you do!?
Haine: OMG! I ONLY ADDED THE NPC WITHOUT TESTING IT, I DIDN'T EXPECT ALL THIS TO HAPPEN!
Alternative ending:
Haine: FFFFFFFUUUUUUUUUUUU-
And that's why you don't simply add scripts or NPCs without testing them, even if the oh-so-sacred eA bug tracker doesn't mention any bugs or the code consists of a mere 3 lines. As a developer you should know that. Especially when it's not that urgent.