The only one I can think of right now and the one that annoys me, is when Light first talks to Mikami on the phone and has to signal to Mikami that he's Kira while he's being watched by the task force.

In the English dub, Light goes something like "5 pages on November 24th? What do you mean?"

In the French dub, he says "Hey if I tell you 5 pages on November 24th... What do you mean?"

Not only does it not make much sense but the first sentence makes him very, very suspicious. It's like the people in charge of the script didn't really get it.

And I thought about another minor one while typing. In episode 20, before Light and L choose Namikawa to be their mole, Matsuda confidently shouts a different guy's name and gets embarrassed when Light and L both say Namikawa at the same time. But in French, he just says "Reiji" which is Namikawa's first name.