Hm, I would say getting almost 100% my first time through playing piano man on medium pro keys, then deciding to switch to hard pro keys. Ended up getting 2 stars from 5. Medium doesn't prepare you at all for hard in this song!
I completed the hard song trainers and practiced a bit and now I can get about 3 and a half stars. 87%. It's improvement.
Another mistake was thinking I could 5 star Caught in a Mosh on medium pro drums. I hate that song, and found out quickly that I hate playing it too.