It's more of a soft limit.

1) You can have up to 31 total buttons; 8 are system buttons and cannot be deleted. If you try to add another button you get an "out of memory" error.

2) It seems the total SIZE of the buttons also counts. For instance I loaded up all possible custom buttons including two huge ones. When trying to download I received that "subscript out of range error". By replacing those two buttons with smaller ones the error went away. Note it was not the physical size of the button, but rather just how much total memory all buttons encompass.

I would imagine this is a correctable problem. Right now it seems things are divided up into strict blocks:

xx bytes for layouts
xx bytes for learned signals
xx bytes for bitmaps
xx bytes for macros, etc.

What it needs is a flexible file format like the Pronto... just lump everything together and ensure it's under the maximum file size.