Recently I have been involved and project which required a web service layer to allow various clients to communicate with our business end.
For this WCF was as always the perfect candidate :D. The service layer had to support both SOAP and REST protocols; for validation of the requests Microsoft Enterprise Library Validation Block was used, having the DTO object decorated with the appropriate validation attributes. All worked fine for SOAP request, if the wrong data was present or if the data was missing from the DTO sent in the request a fault message was generated and sent to the client. But issues appeared with REST request, no validation being triggered and not exception thrown on the WCF service level to point out the issue. After searching on the internet for couple of minutes the solution popped up:

Microsoft.Practices.ServiceLocation.dll
Microsoft.Practices.Unity.dll
Microsoft.Practices.Unity.Interception.dll

Where missing from the WCF service project references. Adding the above mentioned DLLs solved the issue.