Most Common Gear-Matching Mistakes?

I've finally come full circle in assembling a system
that I am pretty happy with. All along, though, I've
been wary of pairing up equipment that may not match well with another link in my system.

What are some of the big mistakes that hi fi enthusiasts
make when assembling a system? And I don't mean choosing
a cheap piece of gear, or a poorly-performing brand. Rather
I'm talking about poor synergies between equipment.

Just want to become better informed, because I know it's
just a matter of time before the upgrade bug bites.