Trailing Slash?

[Jason S Salaz]

A friend and I were trying our hardest to figure out why he could
comment to LiveJournal (and in general, use his domain with LiveJournal
as the server/delegate) by logging in via OpenID via his domain, and I
couldn't.

We finally found out that I was missing a single / in the delegate URL.
(href="...livejournal.com" instead of href="...livejournal.com/").

Is there a reason that this trailing slash is absolutely required?  And
why it (seeming unnecessarily :( ) causes so much grief?