When De Carlo swims out to sailboat at end of film, it is obvious that she is not wearing her dress. Besides, she couldn't have swum out that far in her dress without being dragged under. Yet, in closing shot on sailboat, she's wearing the dress (seemingly dry - double goof)she must have discarded on the shore.
British soldiers did not wear shakos (hats) like those depicted. The shakos of the time tended to be plain black hats with only a regimental number badge at the front.
According to the opening credits the story takes place in the year 1800. Bonaparte was not yet the emperor Napoléon and his attempt to invade England began in 1803. He was crowned emperor on December 2nd, 1804.
Drouchette swam with the Australian crawl, not the breast stroke, which would have been more common then.
There's a scene where Ragan is seen snuffing out a number of candles, as he snuffs out each one, the lighting remains constant.He also puts put one candle twice.
When Rock Hudson leans against his prison cell wall it visibly gives slightly.