Continuity mistake: At one point D'Artagnan reaches the Duke of Buckingham, who is on a hunt. He has just killed a stag and his hands are covered in blood. D'Artagnan rides up and offers him a note from Queen Anne. He wipes his palms with a cloth before he takes the note, but his hands are still covered with blood up to and above the wrists. When he reads the note, he and D'Artagnan take off on foot for his castle where they go into a private room behind the walls. Suddenly it is apparent that his hands are perfectly clean right down to his scrubbed fingernails and, without explanation, the plot-heavy sequence continues.
Continuity mistake: When D'artagnan follows the Landlord's Wife after their liason, and finds her in the street with the Duke of Buckingham, he drops to his knees to apologise for his rudeness, and we hear a splash to indicate he has just kneeled in a sizeable puddle. When the shot changes, he is still kneeling, and he is not kneeling in a puddle at all.
Continuity mistake: About 57 minutes in, Michael York is trying to persuade Raquel Welch to allow him to go to the Queen with the secret message. Early in the scene, something like a ribbon falls to the floor from the table, but later shots show it back on the table without either of them having picked it up.