According to both caffeine's pubchem page and chemspider page its boiling point lies at 173°C and its melting point at ~235°C. How can it melt at that temperature if it already sublimated at 173°C? Or is that the temperature of the triple point? If so, how come none of those pages gives any pressure specifications for that temperature?
I heated up some anhydrous caffeine at home, and at my home atmospheric pressure it appears to melt (not sublimate) and starts creating vapors immediately after (but not before) melting. This is based just on what I can see with the naked eye.