Inverse hyperbolic tangent function atanh() in MATLAB gives incorrect results for arguments outside :

>> atanh(-2)
 
ans =
 
         -0.549306144334055 - 1.5707963267949i

Imaginary part of the result has incorrect sign. Function acoth() is also affected by this bug (since it is calculated through atanh()). Tested up to MATLAB 2011a.

Multiprecision Computing Toolbox

>> mp('atanh(-2)',30)
-0.549306144334054845697622618461 + 1.57079632679489661923132169164i
 
>> atanh(mp('-2',30))
-0.549306144334054845697622618461 + 1.57079632679489661923132169164i

Maple

> arctanh(-2.0);
            -0.5493061443 + 1.570796327 I

Mathematica (link)

atanh(-2) = 
            -0.5493061443 + 1.570796327i