I think the last episode was the revelation that Hopkins is playing God while Harris is looking to free the consciousness. Excellent series about how the conscious works, wasn't expecting it. They basically give the plot away in the third episode when Hopkins explains the Bicameral Mind.
Not exactly a 'huge mistake' as the episode garnered the second biggest viewership draw across the series. They handled it perfectly for their own needs, they knew you were going to watch anyway.
"Worst kept secret since bride's infidelities."
If you can't think of anything nasty just be nice and tell a few gags, In my best man speech I opened with: "They say the key to a great best man's speech is for it to last as long as the groom does in bed, so thank you for lisening and please...