Ah, I've mis-remembered what exactly it was about. Here's the bug:

http://d.puremagic.com/issues/show_bug.cgi?id=9269

Essentially someone added a `version(unittest)` import into a module which had a `pragma(lib)` call for the linker, leading to cases where the user could compile the code if they **didn't** use `-unittest`, but it would fail compilation if the user used `-unittest` but didn't have curl installed.