Lane, Jelly and Lenny send a message home using Wilbur, a homing pigeon. The bird they release through the bars of the window is clearly a different bird from the one shown in the next shot walking across a busy street. It has a different colored head.
When Crout inspects the three prisoners' cell, Lane squirts an air freshener to clear the smell of cigarettes, then thrusts the freshener into his jacket pocket. For obvious reasons, prison jackets are never equipped with pockets.
(at around 61 minutes) The driver reversing the Black Maria is not white-haired Soapy Stevens, but a black-haired double.
At the very end of the film, just as the diamonds falling out the bag to the ground, it is obvious that these are costume jewelry beads, as they have holes through each of them for stringing them with a necklace etc.
The prisoners are able to escape from their cell by removing the pins from the hinges. For obvious reasons real cell doors do not have hinges on the inside.
The Army officer explains to the police that they are not taking the main road to Portsmouth, but using a special route only they know about. It is never explained how Soapy Stevens also knows this route.