Due to the fact that when a product is undergoing tested on an extremely limited release, such as with a handful of employees, it is safe to say that there will be issues that they will not be able to detect until it has made its way into the hands of the masses, where a product will go through all kinds of situations.
The OnePlus 5 is currently experiencing such a problem because according to various reports, there are some owners of the handset who claim that they are experiencing a jelly scrolling effect on the phone, which can be best described as a flickering effect. A video uploaded onto YouTube by Damir Franc (see above, the flickering starts around the 8 min 15 second mark) shows the issue in action.
As to what might be the problem, OnePlus has yet to confirm what the issue might be, but XDA did confirm that they have reached out to the company who told them that they are looking into it. In the meantime it has been speculated that it could be a frame rate issue as it has been noted that VSYNC on the device was disabled. Another theory put forth is that maybe for some units, the display has been mounted upside down.
In any case it remains to be seen whether this is a software issue that can be addressed with a simple update, or if it might be a hardware problem that will require a return. In the meantime for those experiencing the problem, we guess you can keep your phone around and hope a software update fixes it, or you can always try taking it back to the shop.