Designing a half band FIR filter with Scilab
In commerical filter design software, when designing an odd-tap half-band filter your center coefficient (the largest-valued coefficient) will have a value of 0.5. So if one of your filter's coefficient has a value of 0.000002, then that coefficient's value is one part in 250,000 compared to the largest coefficient. That super-small valued coefficient will have a negligible effect on the filter's freq response, so its value can be set to zero. 

In MATLAB, try one of the following:

hn = remez(26, 2*[0 .23 .27 .5], [1 1 0 0], [1 1])
hn = firpm(26, 2*[0 .23 .27 .5], [1 1 0 0], [1 1])


(Notice the '2*' multiplier needed to comply with MATLAB's command syntax!) Examine the coefficients' values and see that none are exactly zero-valued. Now plot your filter's freq response (vertical axis in dB). Next, zero-out all coefficients whose absolute values are less than 0.001 using:

hn = hn.*(abs(hn)>0.001)

and plot the new freq response (in dB). Notice how the two freq responses are essentially identical.