TotalFilm - The Plot Hole: Put simply, "they're digging in the wrong place!"
So Belloq's medallion only has writing on one side, the length of his staff is wrong and Indy is holding the only correct evidence for how to find the Ark.
As it turns out, the Nazis only get their hands on the damn thing because Indy digs it up for them. Why doesn't he simply go home?
Reasonable Explanation: Must be hard to shake the habit of a lifetime. Indy has to see the Ark for archeology bragging rights, even if it means the end of democracy and civilisation as we know it.
Which kind of makes him come across as an arrogant jerk, don't cha think?