This summer at Universal, we had this experience at Rip Ride Rockit, except there are THREE measuring points, plus a metal detector. Our oldest son was measuring just a bit below 51" in socks and solidly 51.5 in shoes. But he was so excited to ride Rip Ride Rockit, we went there at rope drop because he would be tallest at that point. He measured fine at the first two points, so we thought we were good. Then we went through the metal detectors, then we went up to the platform. Right before boarding, they brought out a stick. He stood ram-rod straight and his head, not just his hair, was hitting the stick. It was close, but there was contact. But the CM said he couldn't ride. I questioned it, and the CM just shrugged his shoulders and said, "Sorry."
I really didn't want this to ruin the trip, but it was hard to keep him positive when he was pretty upset, and we had wasted the rope drop..... To make it worse, his mom had taken the two siblings to ET, but we didn't realize that didn't open until an hour after park open, so they went up to the spinning ride in the Simpsons area. She had my phone because RRR has the metal detectors, so we had no way to contact her, and we didn't all find each other until almost 10 am. Our oldest son wouldn't have minded the mix-up if he had actually ridden RRR, but under the circumstances, when he finally saw his mom, he just lost it crying, and hugged her for like a minute. Not a great start to the day.
If only they had bracelets to confirm kids' heights, there would be no issues. You could measure the kids once, when you enter the park, and be all set. I don't get it.