The FontStructor uses something called a "Local Shared Object" AKA "Flash Cookie" AKA LSO to store small amounts of information on your machine – things such as which FontStruction you last edited, or which letter. It's also used for copy and paste.

If you do a LOT of FontStructing this LSO may reach a size limit (by default 100Kb) in which case the Flash player will prompt you to make the LSO bigger.

If you want to delete the FontStructor LSO, or increase the amount of storage available to the FontStructor, you can use the Flash Player Settings Manager. Find FontStruct.com in the list of websites and "delete" the site from the list, or change the amount of storage allocated to it.

Newer browsers will also provide mechanisms for deleting these LSOs.