Who ever made a comment about breathing in and out, this isn't that odd. Breathing in does make a difference in height, not much but if its a grey area (like 1/8 an inch) it CAN make a difference. So for those with kids close in height, make sure they breath in when they are getting measured.
Height checking is NOT an exact science, some may say 40" is 40" but not really. If they just sealed the concrete under the height sign, if its 8AM as opposed to 5PM (someone mentioned that we are taller in the morning which is true), if humidity has caused the height sign to be slightly warped...
Then there is the issue of where a kids head needs to be. I have seen everything it seems, well placed pony tails, sticky hair (gelled hair), baseball caps, etc. WE WILL TAKE THE HATS OFF. WE WILL SMOOTH HAIR DOWN. Hair does not account for height. We need to know where the head actually is. If I touch your sons precious hair don't go running for a manager. And yes sometimes we need a second opinion.
Some may say 1/8 an inch won't hurt my child, and really it won't. There is not that much difference between 39 3/4" and 40". However, if you let someone ride who is 39 3/4" how about someone who is 39 5/8" Where do you draw the line? That's primarily why its taken so seriously.
Last for the person who mentioned running out and buying cowboy boots and stuffing with newspaper. Sometimes that will fly and sometimes it wont. Just be aware. It depends on who is doing the height checks and how observant they are. For me, I would usually notice it. Here (at my local park) we will require the kid to take their shoes off and measure in their parents shoes. (with regular heels), same thing with healys. However, there is nothing wrong with a pair of sketchers.... Usually the rule is anything over a 1 inch heel is re-measured without shoes. Under that though... So if your kid is close DON'T WEAR FLIP FLOPS.
BTW I did not work at Disney, however I spent 7 seasons working at a large local park here, and did more height checks, and was screamed at by more parents

rolleyes1) then I care to remember.
Remember, please be nice to the ride operators, they are controlling that 10 ton train you are riding in...
Jennifer