It seems to be the convention for most fonts to set Sample Text to:
ABCDEFGHIJKLM
NOPQRSTUVWXYZ
abcdefghijklm
nopqrstuvwxyz
0123456789
but the Sample Text field in Pixel Font Editor doesn’t support newlines. I can correct this in FontForge, but it would be nice if Pixel Font Converter just supported this functionality by default.