I had the situation this week where I had developed a provisioning feature for a customer.  I had decided to try and use LINQ.

 

All went well until the feature was deployed to the production  servers.  At this point when the feature was activated all I got was a "file not found" error together with a stack trace pointing at my feature.

 

After a lot of head scratching and generally looking around the site, I finally looked in the 12 hive log files where I found that the missing file was in fact .NET 3.5 - it could not find System.Xml.Linq  - I then realised that the support team had not got round to installing .NET 3.5 SP1 !

 

Installing .NET 3.5 SP1 fixed the problem.