Derangement of Science is one of the most unfairly designed levels, for hiding terrain.
The Lemmings file format (therefore probably also Lemmini) allows to draw each interactive object either in front or behind terrain, as desired by the level author. This may have its merits, it allows some more beautiful eye candy with water, but it also allows such problematic design as in the level.
I will even go further and call hiding exits/traps behind terrain unfair design, which some others here consider acceptable.
Edit: From IRC:
Akseli ponders whether this post should be in spoiler tags.
[17:27] <SimonN> well, if there is terrain hidden, it must be [where it is in Derangement, see spoiler tag in posts above]
[17:27] <SimonN> I recall the level to be interesting even if you know that
[17:28] <Akseli> yeah it is
[17:28] <SimonN> hmm, I will leave the comment then, it makes the level experience better
[...]
[17:38] <Akseli> anyhow whatever the issue is I'm not a fan of it when people say stuff something like their opinion is the only one that's applicable :q
-- Simon