Several factions in this programme have a detachment that cannot be paired because it costs 3 DP and there is nothing left to spend. Those are arithmetic traps, and an attentive player catches them by adding two numbers.
This one is different, and worse, for four reasons.
The arithmetic is perfect. 2 DP plus 1 DP is exactly 3 DP. Every check a player is in the habit of running comes back clean. There is no leftover point and no overspend.
The theme is perfect. These are the faction's two mutant detachments. They buff overlapping keywords. Both promote TZAANGORS to BATTLELINE. One is the cheap mutant package and the other is the heavy one, so they read exactly like a base and an upgrade designed to be taken together.
The collection points at it. A player who has bought Tzaangors, a Tzaangor Shaman and Chaos Spawn owns precisely the models both detachments want, and no models that either detachment wastes. The natural instinct of somebody looking at that pile of models is to take both mutant detachments.
Seven of the nine detachments have no tag at all. A Thousand Sons player builds eight legal pairings without ever encountering a Unique Tag, learns that Detachment Points are the only constraint that matters, and then hits the one case where that is false.
The trap is not that the rule is obscure. It is that everything else in the faction trains you not to look for it.