Other (more official?) sources say that he hit his head while boarding Slave I just after the fight and this is supposed to explain why clones (and later Stormtroopers) are "genetically" prone to bump their heads against the fast-sliding doors (check that famous ANH blooper scene).

If this was caused by a simple head butt, I wouldn't have very good opinion about the alloys the helmet was made from

Shouldn't it be more sturdier? This reminds me of my parent's friend's accident - his car was... bitten by a dog!

Yes, there were teeth marks on the hood! I have another car story on standby, but this would be even more offtopic