The organisation who are responsible for our Data Centre have just advised me that the Testing and production environments are running off the same LAN. To me this sounds like bad practice what - couldn't testing cause a potential performance issue on the LAN which knocks onto the performance of the production app. Am I missing something here and worrying about nothing?
I'd agree it's bad practice, for exactly the reason you state.
Of course the severity of the worry depends to some extent on how busy those networks are, whether they're VLAN'ed in any way, if there are per-VLAN quotas/limits etc. i.e. you could build a very safe scenario such as this but that's far from the default, so you have justification to worry yes.