Covered Pit Bug
Covering a pit changes the trap type
If a PC digs a hole (or generates a pit through some other means, eg. failing to read a spellbook), and proceeds to cover the hole with a plain blanket, instead of a covered pit trap appearing, the trap will magically transform into whatever kind of trap is "appropriate" for that square--that is, whatever trap would be produced on that square if the PC used a wand of trap creation (or a trap was generated natively on that square).
Given that making a covered pit isn't really all that useful anyway, this isn't exactly a high priority issue...