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What is the difference between すみません and すみませんでした (or すまない and すまなかった)?

If it is what the 「でした」 suggests and すみませんでした is the past form and used to
apologize for things done in the past, then what I don't understand is that
even mistakes that I did just now is already in the past (I have already done
it!). Then すみませんでした should be the correct one to use and すみません should be used
only when you are trying to apologize for something that you will do in the
future. But that doesn't seem the way it is used. Can anyone please explain
the nuance difference?

Edit: Here is the thing that is bothering me:  
Anime: キルラキル

> (After complementing her clothes 鮮血{せんけつ} about how good it feels, 鮮血 starts
> to cry)  
> 流子｛りゅうこ｝：「バカ！服が泣くな！ビショビショして気持ち悪いだろうが。せっかくいい着心地{きごこち}だったのに。」  
> 鮮血：「すまなかった」

So he was sorry about an incident (crying) which happened in immediate past
(and he was still crying when he said that), but he didn't use すみません. What
would be the difference in meaning if he said すみません? Or is it totally
inappropriate in this context?
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2014-12-19T16:49:46.310
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2014-12-20T17:35:15.220
2014-12-20T17:35:15.220
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question
nuances expressions
What is the difference between すみません and すみませんでした?
You're largely correct, both of them refer to situations in the past, however,
すみません has has the tendency of being used for immediate apologies (stepping on
someone's foot). すみませんでした is more along the lines of, "Sorry about the other
day."
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2014-12-19T18:19:39.420
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2014-12-19T18:19:39.420
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answer
What bothers you is the ambiguity of ～た form which could indicate both _past_
and _perfect_. And the worse thing is you have no way to distinguish them in
form, in this case.

What you did: `すみませんでした`

> その節はすみませんでした。

What you have done: `すみませんでした` (~ _present perfect_ ) or `すみません` (~ _present_
)

> こんなことになってしまいすみませんでした。  
>  こんなことになってしまいすみません。

Now or future: `すみません`

> すみません。トイレはどこですか？  
>  もし失敗したらすみません。
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2014-12-20T05:42:11.690
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2014-12-20T05:42:11.690
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answer