The short answer to the question is..."Only if they catch you". Probably 95-98% of the time (my estimation), sponsors are ad agencies who certainly don't spend all day listening to the stations. If their client happens to hear something amiss, they will call the agency and the station usually gets the call.
Regarding scheduling, most traffic managers...the good ones, anyway...try their best to keep some type of separation between sponsors. Sadly, more and more companies are leaning away from this as it restricts the freedom to place ads. If a station has too many spots, yes, rates should be raised but my experience has found most sales reps, managers, whatever...don't because they are too afraid to lose the client. Plus, most stations partner with a radio syndicator (such as Cumulus Media Networks) and must carry a certain number of barter spots. Many times, those spots are the same ones the ad agencies place on local stations. It is becoming more and more difficult to separate advertisers so a lot of the bigger companies have simply given up trying.