Add us in as someone who's had this trouble at
Disneyland. When we went in 2003, we measured my son at home, on carpet, and figured he was close to 52 inches without shoes. However, it was a rough estimate, and I really wasn't sure. We went to Knott's Berry Farm first and he wanted to ride Xcelerator. I brought him to the CM there (more to show him he wasn't big enough) and he WAS! He easily hit the bar and rode it several times and also rode the freefall ride (can't remember the name) which was also 52".
We then spent 5 days at DL/CA. The first day we went to the official measuring station and the green light lit up and he got a 52 inch wristband (we still have it!). He rode Maliboomer that day and several other days we were there and never (even without the wristband on subsequent days) was questioned. On our last day, we went to ride it in the morning and "sorry, he's too short". He had 2 pairs of tennis shoes with him (both similar, just regular tennis shoes) so we thought maybe that was the difference. When we went back to the hotel, he switched to his other shoes (I really couldn't tell you which ones he'd been wearing each day). Went back to the park and he was STILL too short. Measuring station was closed for the day, last day there, so sorry. I felt bad for him, but at least he'd gotten to ride it several times before.
I posted this when we got back and had people flame me for trying to get him on a ride he was too small for?!!? What frustrated me (and sounds like it is frustrating everyone else) was that THEY officially told me he was big enough several times, then suddenly he wasn't?! Also, at the ride, I don't remember any bar over his head - it was more like just hold the pole up to him and use the CM's judgement to hold it up straight enough, have him stand up straight enough, and guess if his head was higher than the top. Seemed really subjective to me, which is probably where the problem lies!
We have had this problem at other parks, though. We go to Cedar Point (Ohio) a lot, and when both of my younger sons were big enough to go on Millennium Force (one of their biggest fastest rides) the requirement was 48". They went to Park Ops, were "officially" measured, received a wristband, rode every other 48" ride in the park without question, but when they got to that one, EVERY time (and this was 2 years apart for the 2 kids) they were too short by close to an inch! Didn't take a genius to figure out their measuring device there was off - maybe intentionally just for some added safety?
For those of you planning to bring a tape measure - I agree Disney might not honor your holding it up and showing them that you child is big enough. BUT - I'm thinking you could use it to show them their stick wasn't 40"! If enough people do this, maybe they'll start measuring their own 40" sticks to see that they are in fact 40"!