My most valuable figure is probably
Red Card POTF2
Luke Skywalker (Jedi) - brown vest, indented lightsaber.
In regards to the Boba Fett 300th figure .00 with white background on side panel...
I also own one, but without autograph.
The error was a misprint and they did not catch it in time and enough had been printed to merit the release. I am still looking for specific verification of why, but most information I've read cites the above as being the reason behind the error.