Continuity mistake: When Johnny English and Bough watch Klein and Vendetta escape in the funeral hearse, Bough asks Johnny what they should do, to which Johnny replies, "Watch and learn, Bough". At this point, you can see that Johnny's face is dirty, but when he is driving the truck a few shots later, his face appears completely clean.
Audio problem: When Johnny spills his glass of champagne over the foreign secretary at Sauvage's reception, she says "What in God's name?" but her mouth doesn't match what she is saying.
Other mistake: When Bough tells Johnny to wait for his signal that he's disabled the alarm system whilst discussing their plan in the aeroplane, Bough says, "You then break into this window here." On the subtitles, it actually reads "You them break into this window here."
Other mistake: When Johnny takes his Bloody Mary from Sauvage at the Reception in the Tower of London, he asks him, "Could you rustle up some of those cheesy niblets?" On the subtitles, it reads "Could your rustle up."