GMAT does not impose a size limit on String resources. This allows a user to set a String to a very long string literal, causing general flakiness in the String GUI panel and the script editor.
Specifically, the String edit box stops showing the actual string; the box is blank. The cursor moves along the string as if it were there, but the "End" key doesn't work properly. I'm sure there's other weird things going on also. The same behavior can be seen in the script editor.