Here's what happened:
My friend sold me some land on the 19th, which was her billing date. She knew she'd be paying for May land use fees and that was OK. Since I'd not owned any land the previous month, I was not being charged land use fees, and I didn't expect to be charged for THIS parcel until June, which is the way most people believe it works.
MY billing date turned out to be the 21st, and even though I did not own any land prior to this purchase, my billing date REMAINED the 21st, and guess what?
Linden Labs charged me May land use fees for having owned the land for TWO DAYS before my billing date.
Effectively, they got $250 USD for one parcel, because my friend's billing date was two days before mine.
Apparently, there is no current way to synchronize billing dates so this doesn't occur. In my eyes, that's unfair business practices; it's definitely something that LL doesn't make people aware of in their knowledgebase.
I have filed a JIRA addressing this issue. I really don't expect much to be done about it but it's the only thing I can do.
So...any time a parcel of land is sold, TWO parties will be charged for fees, unless you happen to be lucky and share the same bill date as the person selling you the land (which is not likely to happen). I've proposed they standardize bill dates so that everyone has the SAME bill date and therefore land exchanges can take place with only ONE person being billed use fees, if the transaction takes place on the bill date.
Of course, this means that LL will lose a significant chunk of revenue, so I don't see it being done.