XCode peculiarites -Library can't find file errors

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. I took this one out of the target too and sure enough I got an error about yet another missing library file. I put them all back and got the error about the original file again. Weird. I was stumped for a bit until I took all the libraries out of the target. Finally I got a list of errors which indicated all the missing symbols which is what I would really expect from a linker. So I added the libraries back into the target one by one and when I was finished the build succeeded and I have not seen the missing file error again. I wish I could tell specifically what was going on. The only thing I suspect is that XCode is caching more information in the target than the path of the library file and I was able to clear it out by removing them all.