When Bobby/Barbara is talking to Cliff on the upper deck while he is performing his ablutions the bus is not moving, but in the next frame the vehicle is already travelling at some speed.
When they meet the girls, they not only park the bus on the wrong side of the road (the road sign they hit is facing the wrong way) and all the traffic vanishes.
The road is very quiet and remote considering its the main road from Calais to Paris (pre-autoroute).
When the girls car backfires, the soot only hits the boys faces. Their clothes are clean.
Cliff and his mates are wearing the same clothes for 7 days during the bus repairs/remodelling.
You can't see it clearly but during the waltzing scene he slips, it's in the background so you barely notice.
The scenery at the Austrian /Yugoslav border crossing is not the Alpine scenery that one would associate with that region but must have been filmed much further south.
On the road north of Paris, when the boys meet the girls for the first time, the roadside with olive trees and pale sandy soil looks identical to what they later find just outside Athens.
At the start of the song "We're all going on a Summer Holiday", the scenery on the view through the bus is very mountainous for northern France.
When they first meet the girls, they assume they could be French. Being bus men, they would be able to identify the cars license plate as British in an instant. Also, the car is right hand drive and has a GB sticker on the back.
Before the pantomime in the courtroom, 'Bobbie'/Barbara is seen getting into the Pierrot costume and is present at the curtain call. However, she does not play any part in the performance.