As the polarizing filter is made up of 2 pieces of glasses, you may want to remove the UV filter . I found that it usually cause vignetting at wide angle when i stack the the CPL on top of the UV filter.
1) increase chance of vignetting - i.e. light fall off from corners
2) increase chance of funny colours, i think, not sure about this but er, try stacking uv with ir and you'll see what i mean
3) increase chance of notsogood quality photos