Forgeries of overprints exist, but they are very amateur and can be easily distinguished from the genuine. Much more dangerous are forgeries of postal items where both, the overprint and the cancellation are genuine, but a postal item as a whole is a fake.
A fragment of a letter, franked with 2 din, cancelled with Čakovec 3, 12.IV.41. Private product / forgery. After the Yugoslav army withdrew and when the post office in Čakovec was closed someone stole and misused the canceller. Hence, all letters postmarked at Čakovec on 12.IV.41 or later are fakes.
Letter, franked with 0,50 din, cancelled with Bjelovar 3, 31.IX.41. Canceller Bjelovar 3 is in the private hands and there is no evidence at all that these stamps have been used in Bjelovar.
In addition, forger used a nonexistent day 31st of September :(.
