About the new compatibility rules

[XMuppetSB]

Old GOTOLESS Behavior: In early 2.50 builds, ZASM's GOTOLESS acted as "goto if less or equal" rather than "goto if less."
Old Hookshot Grab Checking: At some point, the pixel where the hookshot checks for hookshot grab combos shifted.
If you set up a screen like this:

Try using the hookshot while standing against the blocks, where the green squares indicate. Either the top one or the bottom one will work, depending on whether the rule is enabled.

Okay, I think I've figured it out now. With the rule on, you can grab onto the combo from the bottom half, but not the top half, and with the rule off, you can grab onto the combo from the top half, but not the bottom half.