Hello everyone, 

I am experiencing something very strange. As a result of a long calculation I obtain combination of functions of the type

Exp[-b t] Erfc(b x- t/x), where Erfc is the complimentary error function, b and t real parameters (b>0) and x some one dimensional variable.
I order to exhibit some symmetry I would like to use the identity Erfc(z)=1-Erf(z), where now Erf is the error function.

If I plot  Erfc(b x- t/x) and 1-Erf(b x- t/x) as a verication, it is clear that things work.

However, plotting Exp[-b t] ( 1-Erf(b x- t/x) ) things get crazy for t<0, where the plot shows some weird oscillatory behaviour and goes rapidly to zero. In the attached picture, the plot shows:

(Red) Exp[-b t] Erfc(b x- t/x)

(Black)  Exp[-b t] ( 1 - Erf(b x- t/x) ) 
 
So I am kind of puzled here ... am I doing something trivially wrong or is it a bug of Mathematica ?

Any help is more than welcome

Thanks