The Top 19 Movie Blunders of All Time:

5) In Indiana Jones & the Last Crusade, Han Solo (Indiana Jones) decides that a simple wooden cup is the Holy Grail, as it is “the cup of a carpenter.” I know many carpenters, and none of them drink out of wooden cups. When I mentioned it to them, most found the idea ridiculous, and faintly insulting.

From the Usemycomputer.com forums.