ってしまった＝"completion" and regret doing that.
 

窓を割ってしまった。
田中さんが来てしまった。
ＣＤを買ってしまった。

ってしまった means "completion" and regret doing that.
or "completion" with negative feeling.

先輩のほうがこぼした時、「○○牛乳こぼしちゃはった 」言う風に言わんとやっぱり失礼なんけ？

It looks more like they`re asking about it being used for cleaning or something like that to me. The example sentences seem to be throwing everyone off as they`re unrelated to the question, really.

I can`t think of an example off the top of my head where しまいます is used on it`s own, but think of the pattern しまっておきます. Or しまってあります.
They are used all the time in relation to cleaning and putting things away.
ホウキはクロゼットに片付けてある。 = ホウキはクロゼットにしまってある。

Or at least it does around here. I would feel really weird using the first sentence.

It's funny because whenever I heard "しまっとく" or "しまってある" I always pictured the verb しまる instead of しまう! Upon looking it up I realize I was mistaken though.

I'm guessing your post was directed at Supperman, but I learned something nevertheless.

No matter what, thanks for all the inputs. :)

I always hate being reminded of the fact that I suck at speaking Tokyo slangだピョン

LOL Supperman must be the one who wrote my textbooks :)

In your first post in this thread, you neglected to mention the possibility of implying regret over something occurring. You just mentioned finality. I think this is what masaegu was getting on your case about.

I am sorry...I didn't want to sound as someone who was criticizing Masaegu. Far from my intentions to correct a native speaker. I do too help people online who are studying my language and I know how annoying is when a student doubts my words.
The problem is that the books Masaegu mocked in his message are textbooks that every student uses (I don't use anything special, Genki, basic grammar dictionary and others). So he can make fun of me (and no, kakkoi meaning I got it from 2 crappy programs like ALC and Ricaichan, not from textooooks) for my knowledge, but my knowledge is something which is spread amongst many students coz those textbooks are used by many many thousands.

I think while Tokyo-ans might understand what I wrote it isn't Tokyo slang as far as I know. It's more Kansai-ish I think. ~してはった is 敬語. ~しちゃはった is like an original grammar that I made up that sometimes gets a laugh. It's like you are saying that someone accidentally did something but 'gracefully'. I guess a more 'grammatically' correct way of saying it would be しちゃってはった, but it's too long so I shorten it.

People in Tokyo wouldn't even understand my Japanese unless I tried to speak standard :o .

By the way, are you saying "ピョン" to be festive (lol pardon the phrasing) because of "usagi doshi"? I used that on my new years cards and it seemed to be a hit.