Today in my history class, the professor gave a lecture on millennial fears as the year 1000 approached, and why the evidence is conflicting. The main thing I noticed during her lecture was that when she wrote on the overhead, she spelled 'millennium' with only one 'n'. This is one of my pet peeves, so I will explain here why 'millennium' should always, always be spelled with two n's.
'Millennium' is derived from the latin words mille, meaning 'thousand', and annus, meaning 'year'.
If you take out the relevant n, you're left with mille and anus. The latter, as any seven-year-old teller of astronomical jokes knows, means 'asshole'.
So 'millennium', with two n's, means 'one thousand years'.
'Millenium' with one n can be a synonym for 'fandom'.