What are some of the weird furigana that people have seen?
By "weird", I mean pronunciations that you never hear in Japanese.

For example I was reading a Japanese cooking magazine,
and came across the word 胡麻山椒 (こまさんせう).

The magazine broke the furigana up as follows:

胡 = こ
麻 = ま
山 = さん
椒 = せう

This is probably the only time I've ever seen せう used as furigana
for a single kanji.

Anyone else with similar examples?

Isn't it ごま?

I see せう a lot because it's just the pre-WW2-kana spelling of しょう. It's not that common to see it in modern publications but if it's quoting an old source or a place name I guess it's possible. (In any case, it's not pronounced "seu".)

山椒　＝　さんしょう, with the 椒 being the しょう

IIRC, older writings often had せう instead of しょう, as well as さう for そう, and a few other things.
This is also why we don't see ひ in place of い in certain words too.

I haven't read anything with old-style writing in a looong time, so I'm not 100% on what switches to what, but my guess is that it's not so much "weird" as it is just "old."

I enjoy some of the things that were listed in the Harry Potter thing.
夜の闇（ノクターン）横町 in place of Knockturn Alley (nocturnally.)
and then something for the Knight Bus.
But that's not so much "funny" or "weird" as it just "punny."

If it's not pronounced seu, how is it pronounced?

しょう.
http://en.wikipedia.org/wiki/Historical_...rthography