Case 1: the secret tunnel

In the Beta, you can't go in the tunnel.

Case 2: another secret location

In the Beta, this animation is displayed while running up the slope.

Case 3: Act3's future

In act 3 of Palmtree Panic in the beta, there is only a bad future with different looking palm trees and star plants. As this is unique to act3, it is most likely that the act was still in the process of being finished. Dr Eggman's Egg-O-matic has claws instead of bumpers.