Wow, too many to list. I'll just stick with the ones that spring to mind...
I thought Ackbar was called Ackbarn
I thought Tarkin was called Tock (you can't really hear the '-in' when Leia says it)
I thought the Emperor was like 1000 years old, not 80 (but who didn't?)
Same thing for Vader
I had no idea that the guy next to Obi-Wan and Yoda at the end of RoTJ was the same guy from five minutes previously
I thought the stormtroopers were robots
I didn't realise that Wedge was at the end of RoTJ, I thought it was just Luke hugging some random pilot. If you remember, we don't actually see Wedge with his helmet off until that moment, so I didn't recognise him.