When I input a complex 4-component vector, the mathematica throws an error:

After then, I obtained the InputForm of the errorneous result as follows:
{(E^(I*\[Phi])*(px^2 + py^2 - Sqrt[px^2 + py^2]*\[Mu]) + E^(I*\[Phi])*Sqrt[(px^2 + py^2)*(px^2 + py^2 + \[CapitalDelta]^2 - 2*Sqrt[px^2 + py^2]*\[Mu] + \[Mu]^2)])/((px + I*py)*\[CapitalDelta]),
(E^(I*\[Phi])*(Sqrt[px^2 + py^2] - \[Mu]) + E^(I*\[Phi])*Sqrt[px^2 + py^2 + \[CapitalDelta]^2 - 2*Sqrt[px^2 + py^2]*\[Mu] + \[Mu]^2])/\[CapitalDelta], Sqrt[px^2 + py^2]/(px + I*py), 1}
After I re-type this InputForm result, then there is no error and the exact same expression is recovered! What happened in this process, and why there was an error in the first try?