I noticed this one when upgrading some projects in CPLAT (a nice cross-platform framework) to XCode 2.1 from 1.5. After creating the source tree for CPLAT in the XCode preferences the compiles went well but the link phase gave an error about a missing library. The files seemed to be present and in the target so I tried taking it out of the target to see what would happen. (I was hoping it wasn't even needed.) Instead I got a new error about another library being missing, I think it was expat.