level 10 has an invisible exit.
level 15 has invisible terrain
In both of these cases, those are the entire point of the level, and should not be changed. People who really don't like this are free to skip over those levels.
For the record:
Level 10 is very similar in nature to Tricky 28, but larger, gives more skills, and the exit is invisible instead of hidden in terrain. The whole point is to hunt the exit; if it was made visible, the entire point of the level is destroyed.
Level 15, you are likely to discover that invisible terrain exists purely by accident. From there, looking closely at the level layout gives hints as to exactly where the invisible terrain is. It is about as far as you can get from a blind, unfair game of "guess where the hidden terrain is!"
Also, did you include the two former levels that were removed? I remember that at least one of them was a fairly decent level. :)