A long time, I had a problem getting into Spain with an official US government employee passport; I was invited by the Spanish government to give a talk. When I got to Spanish passport control, I was told that I needed a visa! If I had a regular passport, I didn't need a visa. I was told to go to the office for further processing.
Fortunately, I was travelling also with a regular Canadian passport and I went to a different line at passport control and got in no problem!