When I first bought Mass Effect last September ( started late ) I didn't know what The Reapers were. At first when I saw sovereign I actually thought this was a Reaper ship not an actual Reaper. I thought The Reapers were a species of machines similair to The Geth and that Sovereign was one of their ships. I think the biggest plot twist for me in the first game was when I realised that Sovereign was an actual Reaper. Since I hadn't paid attention to Mass Effect before then it was actually a good plot twist.
What was your "I wasn't expecting that" moment?