If Natural Math finds an error, it will spit out the part of the lines it was able to process, and then follow it with a kind of descriptive error message, including a rough idea of which line number it was in the .nat file where the error happened. This same error will also be written on the command line at which you ran naturalmath.
Here are examples:
These errors were inserted deliberately.
These errors were inserted deliberately. ^ Error: what's this: `These' just before line 614
16 * x - 1 + 2 over (x + yy) - 11.2235 + 24 / 13
1 + 2 over (x + yy) - 11.2235 + ^ Error: what's this: `yy' just before line 619
(1+2 ^ Error: right bracket missing just before line 622