I see you prefer the explanation to be in Polish, but I'm afraid I can't manage that :)

I've hidden my first answer because after further research and thought I've reached the opposite view. "Absence of mistake or accident" means "absence of mistake or absence of accident". The "or" expresses alternatives; it doesn't means "nor" here.

This is an American legal formula. In your case it comes from Wisconsin; the example I'm going to quote comes from Indiana, but the issue is the same.

"Absence of mistake or accident" expresses two (not one) of the purposes for which evidence of the defendant's bad character, and specifically previous crimes, may be admissible.

A defendant may argue, among other things, that the alleged offence was the result of a mistake (e.g. he/she didn't realise that the white powder was cocaine) or an accident (e.g. the gun went off by accident). If the prosecution can show that the defendant was experienced in the use of firearms or drugs, these defences are less likely to be successful.

It's not impossible, in theory, that the defendant may argue both mistake and accident. If so, the prosecution must rebut them separately: it must show both absence of mistake and absence of accident. But normally it will be one or the other, and so "absence of mistake or accident" means that that the prosecution may introduce evidence of previous criminal acts to rebut either of these defences, but not (necessarily) both. So "or" expresses alternatives: either/or.

"5. Rule 404(b) restates the general rule that evidence of bad character (in this case, prior crimes and acts) is not admissible to prove that the defendant has criminal tendencies.
6. However, Rule 404(b) provides that such evidence may be admissible for any of nine other reasons:
--Motive
--Opportunity
--Intent
--Preparation
--Plan
--Knowledge
--Identity
--Absence of mistake
--Lack of accident
[...]
11. INTENT, ABSENCE OF MISTAKE, LACK OF ACCIDENT (MENS REA). Other crimes (usually several similar ones) are admissible if:
a) The defendant denies some aspect of mens rea by claiming he did not intend harm, acted under a mistake, or that the harm was an accident."
http://law.indiana.edu/instruction/tanford/b723/outline/07-c...

So there you are: mistake and accident are separate claims, and the prosecution doesn't have to establish absence of both unless the defendant claims both.

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Note added at 7 hrs (2014-06-14 12:49:43 GMT)
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The expression doesn't refer to showing that both were absent, it refers to showing that one or the other was absent, whichever one the defence alleges. If the defence alleges a mistake, the prosecution only needs to rebut a mistake, it doesn't need to show absence of accident as well, unless the defence also alleges accident.

They are alternatives, because they are treated separately. The point is that this refers to the possible reasons why evidence of previous wrongdoing may be admitted. It may be admitted to show that the offence was not a mistake, OR it may be admitted to show that the offence was not an accident (or to establish motive, or opportunity, or identity, etc.). It may be admitted for any one or more of these reasons. So it's a matter of motive or opportunity or intent or [...] absence of mistake or absence of accident. Or any two or more of these.