Given a correlation ID, Merge-SPLogFile is one of the best ways to track down a SharePoint error as this cmdlet searches for this ID across the ULS of all servers in the farm. You can even specify other filtering criteria, such as Start/End time, category, etc.

Today I used this cmdlet on an error ID reported by the user. The cmdlet however did not return any record. I manually searched through the ULS text files on the server and was able to find the error. What’s going on!?

As it turns out, the correlation ID specified to this cmdlet has to be UPPER CASE!! The cmdlet returned records as expected after I changed the casing of the correlation ID.

I guess this was alluded to in TechNet:

Specifies the correlation ID to filter on. The type must be a valid GUID, in the form F0BB0790-4323-A153-096F-ABCDC80E24D4.

But seriously.. who would have thought?!