Lets look at examples of each.
Acne is well known for its tendency to create persistent marks on the face. If these are typical flat or brown spots with no fibrous texture or pitted quality, most likely they represent post-inflammatory hyperpigmentation rather than true scars. Over months these 'blemishes' will fade, then become undetectable. Topical medicines like Retin A can speed the process. Another common example of hyperpigmentation following minor skin injury is that which follows the clearing of eczema or flea bites.