One of the issues with the MacBook I bought has been getting everything connected. I have my keyboard/mouse, two LCDs, and speakers that all need connected. Out of the box it worked (albeit with some pain), because I bought the last-gen MBP that wasn't all USB-C, so I had two USB-A ports, an HDMI port, and two "Thunderbolt 2" ports. TB2 is mechanically and electrically compatible with mini-displayport, and my monitor came with such a cable that I use in the other direction (full size DisplayPort connector on the PC to mini-displayport on the panel). So I flip that cable around for the main LCD, HDMI for the other, and I'm all set. Except when I want to use my desktop at night, I have to undo this whole mess.
So I very quickly bought a pair of mini-DP to DP cables on Ebay - I'm trying to avoid falling into what seems to be an Apple user habit of spending way too fucking much on gear. Unfortunately, I may have gone a bit far in the other direction. The cables turned up on Wednesday, and one of them worked flawlessly, but the other not so much. I thought at first that my second panel doesn't like the DP output from the Mac, but when rearranging cables to clean things up (run them behind instead of in front of the monitor arms, etc) I swapped the cables around and now it's the main monitor that misbehaves.
So yesterday morning before work I wrote the seller explaining the issue, and shortly before the end of work they got back to me... asking to see a video. So sure, I'll play along, and sent them a video. A reliable way to get the monitors to resync is to unplug the mag-safe cable, which shuts off external monitors (and seems to put the Mac to sleep instantly?). Plug it back in, press a key on the keyboard, and with my DP cable both monitors come up in a few seconds. With the other one, sometimes it comes up, sometimes the monitor powers on and then shows "DP: No Signal".
Not to mention that I noticed both my cable and the good one will negotiate at 120Hz, whereas the suspect one will only negotiate at 60Hz. I'm fairly certain it's defective - will just have to wait and see what the seller does about it. Even if they won't fix the problem, I'm still a tiny bit in front over buying decent cables... except I don't have "decent" cables. :(