Pretty Woman (1990)
Everyone was so distracted by Julia Roberts and Richard Gere getting hot and heavy in Pretty Woman that they missed one important detail. When the scene opens, Vivian is wearing a long nightgown. By the time it's over, her nightgown has become significantly shorter.
Julius Caesar (1953)
This 1953 history drama forgot to leave behind its '50s garments. At the time, bullet bras were the most common type of women's undergarment. Unfortunately, since the movie takes place in 44 B.C., the bullet bra was a bit ahead of its time.