I just noticed that under the heading of strings, the Help file states:
"String variables default to 10 bytes for chips with less than 16 bytes of RAM, 20 bytes for chips with 16 to 367 bytes , and 40 bytes for devices with more RAM than 367."
The middle clause strikes me: if a chip only has 16 bytes of RAM, then it's going to be pretty difficult to store a string 20 characters long.
A good point. I can clarify the memory constraints within the help file and republish.