The problem started because I wanted to switch a test against a certificate located on the disk to using a certificate located in a certificate store. For this test I decided to pull the certificate by serial number so I snatched the serial number from the certificate properties easily enough.
I tried this as-is and it wasn't working. So I tried removing the spaces, still no joy. So I grabbed the entire list of certificates on the machine and found the certificate to verify the serial #. The results showed the serial number in upper case, so I quickly popped a .ToUpper() on my key. NO-JOY. Ok, WTF. So I copy the serial number out of the watch results in the IDE and try again, it works.
But I couldn't leave it at that. What was wrong with my original value? I restored the changes and created a second set of checks (See first screen) and started cursing my monitor in disbelief. I lined them up, reversed the order they were called in the Find() statement, and cleaned and rebuilt the test application several times. String.Compare returns a match, .Equals returns not equals. The call to the store to Find returns a result for only the one value, regardless of which order they are executed.
The only possibility: copy and pasting the original value from the certificate dialog resulted in some unprinted Unicode character being embedded in the original string. (serial2) String.Compare ignores it, but .Equals does not. (The test1/test2/result check was just me testing my sanity that .Equals on string wasn't doing a reference equals.) To quote Sir Arthur Conan Doyle: "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."