In Indiana Jones and The Last Crusade, when Indiana Jones steps on the letters to spell out the God's name and he accidentally steps on J, which was the wrong letter, he falls and grabs onto other letters to not to fall. However, the letters he grabs are L and Y, which are also wrong letters. According to the clue, these letters should also have been collapsed but these do not? why?
Well, I think the answer of this question is in the second clue, which is:
Only in the footsteps of God will he proceed
This clue means that he has to step on the letters that spell out the God's name. When he puts his first step on the wrong letters and falls, he grabs the wrong letters with his hand (not by foot). That is why these letters
L and Y do not break.
Besides, when he reaches the letter O, he also accidentally steps (about one-fourth part of his foot) on letter P which was just before the letter O. Since it is wrong, it falls down. These are some assumptions we can take from the second clue, and I do not think that is a goof.
Previous answers assume that only those letters which were on the path across had supports underneath them. Nonsense. For a floor that large to stay up, lots of letters had to have supports under them. All that is necessary is that those supports which aren't on the true path eventually lead to dead ends, where the next support isn't reachable by stepping. When Indy fell through, he was simply able to grab the top of a letter on one of these off-path supports.
And another interesting fact from the site is this mistake:
Jehovah is spelled Iehovah in Latin. However medieval languages (Latin or vernacular) had neither official rules nor the letter 'J'. It was not until the sixteenth century that the French humanist Pierre de la Ramée proposed to use the 'J' as a separate letter distinct from the 'I'. The 'J' could therefore not have been a trap at the time the test was constructed.