Well yes it is bad to mix different pills only for the fact that you don't know the exact contents of two different types of pills. For instance 1 type of pill could contain DXM and another type MDMA. Either on its in multiple doses would be ok (although 1 not what your looking for), but mixing DXM and MDMA in larger doses can lead to a hosipital visit or death.
Generally though, im sure hundreds of people mix pills every weekend, with no ill effects. But if on the rare chance you get two pills with different contents that don't go well together, you know the rest....
So i guess there is some truth to that rumour, my advice: only mix pills that have known suspected contents on pillreports or you have tried/tested for yourself.
EDITED: too slow