@retardware: I mean a 16-bit segment top-down segment descriptor loaded with a limit of FFFF to FFFFFh. That descriptor will load into the descriptor cache without any problem(e.g. MOV DS,AX), but when software tries to access ANY address(doesn't matter which one), it results in a #GP(0) fault because only accesses larger than FFFFh(or larger when set between 10000 and FFFFF) are valid and offsets larger than FFFFh are invalid due to it being a 16-bit segment descriptor(B-bit is cleared).
It might theoretically be used for stuff like trapping ANY access to any software kind of memory allocation, with the software in the kernel handling the real access of said segment(think in terms of, say, emulated video card memory?) for protected-mode software using a specific segment selector and descriptor instead of through Paging(a segmentation-based method of doing that, instead of Paging-based method).
Edit: @crazyc Thinking about it, doesn't windows use it to implement lazy stack allocation and remapping(or maybe one older version of Windows)? I remember having read that somewhere.