I had the opposite problem, the sign was wrong for the posted height, the county poured a layer of concrete over the old concrete and then never updated the sign to show the lower overall height.
I crawled up to the overpass and things didn't look right, so trusty grade rod in hand, I got out and measured it, sure enough, 8 inches lower than the posted height and I was then too tall to make it under. In the meantime plenty of commuters came along and I was blocking the road and it took the police to route traffic so I could back the mile up and out to avoid hitting the overpass.
I usually try to drive the route empty I'm going to take loaded so I can measure things when nobody is around and I'm not blocking anything later on, that time I didn't do that.
The only signs I somewhat trust are those posted on the interstate, all the rest, especially those done by the county I try to measure especially after road work has been done recently.