Just saw this in a Juniper Document
Never thought about that. Anyway, I never saw a functional problem with it. But I guess they created the warning for any damage created on the chassis caused by those SFP’s. (they are a big bigger then optical sfp’s).
Do not install a copper transceiver in an access port directly above or below another copper transceiver. Internal damage to the access ports and switch can occur. We recommend that you either use the top port row exclusively, or the bottom port row exclusively, for installing copper transceivers.