Hi,
Another unknown condition not described in the datasheet.
When using a SYSCLK of 250,000, and DIV/2 mode with 65,535 counts the lowest frequency can be 1.91Hz
With PIC16 there's usually an overflow interrupt which can be used to set a flag which overrides the calculation, otherwise, and I'm guessing, TCBn could return a very small period.
.. but maybe it sticks at 65,535, or 0, which would be nice, but alias there's no documentation on it.
Cheers,
Andrew