Yes it is a common problem. I have seen advertising covering cameras many times. Another problem that occurs alot in retail stores is having hanging signs next to air conditioning ducts. When the air blows the sign it sets the alarms motion detectors off. Or, the signs block the motion detectors. We had one client that had the ATM machine ripped out and the alarm didn't go off because of hanging displays blocking the detectors (glass break and motion). They actually painted over the glass break detector as well.
I have an ongoing battle with our visual/layout people on this. Luckily, most of the store-level people are usually pretty considerate when we approach them about it, and find a way to make it less of a problem. Unfortunately, to the corporates we don't exist. Or at least, it appears that way with how little attention they pay to us. *sigh*
"I don't do judgment. Just retrieval."
"The true triumph of reason is that it enables us to get along with those who do not possess it."