Now this is a neat trick. When using the gold granny in towns to refill your magic, she fills it up completely. Due to a glitch in the game, you can have unlimited refills in one visit. As long as the gauge in filling, you can use as much magic as possible, but once the gauge has filled completely the unlimited refill is over.
This is handy for towns without life refills or ones in which you need magic to gain access to other areas in the town.
*NOTE: This unlimited magic trick does not work with red/blue magic refill bottles found on the field. The red magic refill bottles refill 8 squares, so technically you would only be able to use 8 squares worth of magic during the refill. For example, Shield magic uses 16 units of magic, or 1 box worth. It all comes down to math, and you thought math would never help you out in something like video games. Math is power!