Do your homework.
If a race advertizes a 5K then the race should be a 5k. Unfortunately, there are still a few races out there that don't get it. These events are usually a fund raiser for a local charity and they don't do their homework.
As a runner you should do your homework as well. Unless you want to support that particular charity/event and don't care about the distance you should check the application - see if the course is USATF Certified, see who the timer or group organizing the race. Go online and see the course on the USATF website (a recently measured and certified course may not appear on for several weeks). If stll in doubt, contact the race director.FYI- A Certified course does not gurantee that the race director, volunteers, or lead vehicle will maintain it's inegrity.
In addition, check to see that the race is SANCTIONED by USATF or a Member Organization of USATF (RRCA, NCAA, IAAF, etc). A runner can not set an American Record (Open or Masters) if the race is not Sanctioned. Most events think that they only need a Sanction for insurance purposes - and therefore many events organized by municipal recreation departments never get one (self insured). An organzing group can get a Sanction without the insurance for a much reduced cost.
Do your homework.