Insisting on daisy chaining monitors by plugging into the out, despite knowing they are parallel.
I've got a DI box where the in and out/link are parallel, so it doesn't matter.
I've also got another DI box where the in and out/link are isolated, so it DOES matter.
A few years ago, I was helping a client move into a new office. At each station were two Ethernet jacks. They wanted to know which one to plug the PC into, and which one to plug the VoIP phone into. I told them "plug the phone into the top one" even though both jacks went to the same VLAN. I've long ago found out that when people are doing things they don't understand, it's easier to tell them what to do rather than give them a choice, even if the choice doesn't matter and leads to the same result.
If they are seeking to learn, then you can explain and give them a choice. Otherwise, let them continue in their blissful ignorance.
Running in to in and out to out -- even if it doesn't matter to the system -- is probably a good idea because SOMEONE will point out the "problem" (or worse, try to fix it) and you just don't need that hassle when you're busy trying to do something else.
(Now if you really want to confuse someone, use a passive DI backward to connect a balanced out to an unbalanced in.)
