Wired/Wireless Simultaneous Connections

Will having your wired and wireless connections active at the same time cause problems with a network, and if so, what issues can this cause?
I was told: You should always be aware of whether you are connected to the wired or wireless network, and use only one network connection at a time. Your laptop can also be connected to both networks simultaneously, which can cause network instability and slow connectivity.
Thanks in advance!