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I have seen crossed unit line sets, crossed control wiring, but the best one was a power feed from one tenant's breaker panel to a different tenant's condenser.
The tenant whose breaker panel was supplying the power for the condenser did not have a/c in his own space and had effectively been paying his neighbour's ac bill for quite some time.

Never seen mixed up pipes, but I have run across many mixed up control wiring, on a 20 year old property got a call that their unit was not cooling, get there and find outdoor unit running and icing up their indoor unit coil clean and dry, went downstairs to the apartment below and their unit is off but the coil is frozen over. had worked this way for over 20 years and nobody notices till the new resident did not want to use their A/C.

Did it once, caught it when I hooked the vacuum pump to the first unit I hooked up. Mixed up liquid lines.

Ive have seen this done before aswell, but we figured it out before startup. I have always wondered how they would run... assuming everything was exactly the same (pipe size, length of run, uit capacity, etc.) and they both ran at the same time, i suppose its possible they would work... interesting.

I will admit this now; many many years ago, I as an apprentice, hooked two side by side split systems same size, liquid line to one suction to the other. They worked fine, until someone shut off one of the systems. Then it became apparent that there was a problem. It was an easy mistake to make. Easy to correct. I learned something.

A Shared Split System?

We have taken over the service at a local retirement center. About a year and a half ago another contractor installed about 80 1.5 ton heat pumps and they've been having a lot of problems.

We've had compressors and reversing valves go on a few of them. A few weeks ago a tech found one that no matter what the head pressure was about 500psi and blowing reliefs. Ordered a new txv and reversing valve. They cam in yesterday and a different tech went to install them. During charging, he was experiencing very crazy things. after about 14# of 410, he couldn't get any kind of suction or head pressure (normally these units take 8-12#).head pressure at 150, suction at 50.

Today he went out with our service manager, thinking there was a plug, or bad compressor, or something, they eventually found the cause.

The installing company mixed the liquid lines up with the unit next to it.

wooops