I heard this one from a friend who's currently studying law - His professor gave it as an example of the fact that it always pays to read the full Act. I don't remember the exact dates and names, but the gist of it was that in about 1850 in England, in the middle of some very long and tedious land bill, one of the civil servants inserted a section something like the follow:

27(4)(c) Furthermore the marriage of draughtsman John Smith, of 27 Uffington Place, is hereby annulled and declared void, and possessions of the two spouses shall be sundered between the two, in proportion to their yearly earnings, and no further responsibility shall rest with either party.

The bill was passed and signed into law before the section was noticed - so the guy got his divorce. Also, since it's illegal under Common Law for the parlaiment to force two people to marry, there was nothing anyone could do about it, even if they repealed the act.

Bizarre one number two. In Ireland, there is a law still on the books that makes it illegal to impersonate a witch. There's no law about being a witch, but you can't impersonate one, apparently. The media drag this one up every Halloween and chuckle about it.

And number three. This last one is probably anecdotal, but anyway: I very much doubt you'd get away with it, as I think it's only a college bye-law - but apparently in Trinity College, Dublin, there is one day of the year where you can legally murder. The law, dating from some time around the year dot, states that a person who stands on top of the campanile (a big phallus-type monument thing) and shoots a Catholic with a bow and arrow will not be prosecuted so long as he remains within the grounds of the college.