Two battleships fire many broadsides at each other (some hitting), but during the boarding scene no damage is visible.
When William Shaw is in the captain's quarters trying to decipher the treasure map, he looks at a log book dated 1670, though the opening titles indicate the movie takes place in 1668.
At about 1 hour and 13 minutes during the film, when Morgan Adams tries to start a fire inside the cave, they first saw a crab on a rock, then it disappeared, then it appeared on the rock again.
Morgan's ship, being pursued by another, is supposed to be running at full sail, but a wider shot that includes some of the surrounding water reveals it to be motionless.
The tattooed man's eye heals quickly.
When Morgan is fleeing on the coach, the British ship opens fire on her with cannon which have absolutely no recoil and fire exploding shot.
During a bar fight, when Morgan and William swing from the chandelier, there is no movement from the candles which should be flickering - but don't. The candles are electrical lights.
After Morgan finds the gold and jewels in a cave and she handles them and drops them into the pile, the sound of metal is not present. What should sound of metal, sounds of plastic hitting each piece.
As earlier, at the commencement of the final ship-to-ship battle between the two ships, though they are both sailing at relatively high speed, with one rapidly approaching the other, at the moment firing commences, it is clear that they are both standing completely still, as if at anchor.
In the scene where Morgan and Shaw are running from the town after fighting with Dawg brown, a plastic garbage bag (most likely filled with acetylene for an explosion) can be seen rolling off the porch.
As Morgan swings on the ship's rigging lines, she can be seen to be wearing a safety harness.
When Shaw is below decks on Dawg's ship he shoots the lock to release the chain on the treasure. However, just before he fires it is clear that there are two padlocks on the chain, both have their keys in them.
When Dawg holds Mordecai at knife point, it can be clearly seen that the knife is a prop with a collapsing blade.
During the opening titles when Morgan (Geena Davis) is riding the horse to save her father she is wearing sandals - however when she kicks the pirate manning the dingy she is wearing white trainers.
Through her scenes on Cutthroat Island, tan lines from sunglasses are visible on Morgan's face.
In Port Royal there are several scenes with the Union Jack flying, which was not adopted until over 110 years later than the setting of the film.
The Finnish flag on the shop front window was not in use at the time the film was set.
When Morgan falls, we see her wearing stockings with suspenders (garter belt). These were not worn by women until after the First World War. She would have had tied garters around her thighs, in this period.
During the final battle between the pirate ships (at 1:38) Geena Davis shouts out, "Riflemen prepare!" as her crew prepares to board the other ship. Rifles were not in common use until the 19th century, and would not have been used at sea in that period.
When Morgan takes her dagger to shave her fathers head the sound is that of a switchblade.