The Crypt of Shadows is an example of the difference between fair and unfair traps.
This is the start of a new section of Keep on the Shadowfell, which can be broadly described as "the tombs". The flavour enemy here is the undead, and the climax of the area is a confrontation with the ghost of Sir Keegan, the Keep's last commander.
The Crypt of Shadows leads into the tombs with a short maze of corridors, which the players enter from the east (top of the map). "Strange designs" are inscribed on the floor at intervals. Stepping on one of these designs triggers a "throat-tearing scream", which summons the zombies who are waiting at the south-east and north-west of the area, and also subjects whoever tripped the alarm to a fear effect which can send the unfortunate player running straight into the arms of the undead.
There are some problems with the encounter. One is that players will almost certainly clear out the zombies in the south-east before monkeying with the designs, which takes a lot of the sting out of the set-up. Another is that the southernmost inscription is redundant and can be safely walked around.
The biggest problem, though, is in the description of the runes. The module says only, "Strange designs are inscribed into the floor," and then provides an unhelpful picture of what the designs look like. A DC 20 Arcana or Religion check reveals what the runes do, but that can be a tough roll for even a trained second-level character. (A best-case scenario sees only a 55% chance of success; more realistic chances for trained characters are 40%.)
No rules are provided for "defusing" the runes, although that's a fairly simple house rule for the DM to come up with. Instead, players are expected to jump across these obstacles. Experiments with three parties suggest that new players simply won't think of this; the runes are 10 feet across and on the battlemap it looks too far to jump. The DM will probably need to mention that 10 feet is a jumpable distance.
Jumping them turns out to be quite easy - DC 11 with a running start is 50/50 odds for even the most inept character - but with five characters the cumulative percentages mean that you'll almost certainly have someone fail during the jump. In most situations, that means the party split on either side of the rune as the undead come surging forwards.
This isn't a fair trap. There are several reasons why. The first and most obvious is that it doesn't make sense. Regardless of whether this is an ancient trap set by the keep's original owners or a more recent defence installed by Kalarel, it's haphazard. There is no particular scheme to the placement of the runes - they're not defending anything or placed with any kind of intelligent design. They have the effect of encouraging players to go south before they go north, which is a good thing, as I'll come to in later posts, but they feel random and unfocused both in the larger picture of the keep and in the smaller scale of this encounter.
Another reason the trap is unfair is that by requiring everyone to jump the trap plays on the weakest member of the party; it's punishing players for specialising rather than being generalists. In every other context specialising is the right decision in D&D so here it feels like players are being made to lose because of how often they win.
The third reason that it's unfair is that players aren't provided with the necessary information. There is nothing about these runes to suggest what they'll do, which makes it hard for the players to make meaningful choices about how to approach them. The only available information comes from the Arcana and Religion checks, which are punitively hard considering how vital their information is.
On each occasion I've seen it run, this encounter has been completely underwhelming. It's a definite low point in Keep on the Shadowfell and if the whole module had been like this it's a wonder how anyone could have got past the first session.