You need to work with clips with a different fps than the one on your timeline (always my case: 50fps/25fps which gives you on demand nice slowmo - I use them intensivily on my work), and even the bug might appears on peculiar config/videocard only. This important info for those experiencing this issue has not been provided by the community (except me here and there) which apparently and mainly prefers to make useless comments than be helpful. Indeed. This bug is listed on Neat video known issues as follow for two days:
"FCPX 10.2: A new problem (possibly a bug in FCP X) related to use of Neat Video along side with transitions between clips has been introduced by Apple in FCP X 10.2. This problem shows up when the frame rate of the project is different than the frame rate of the clips involved.
For example, if a cross-dissolve connects two clips while instances of Neat Video are applied to each of those clips and their frame rate is different than the frame rate of the project, then the transition area may render incorrectly. Also, a part of the second clip may even go black, apparently because FCP X serves incorrect frames to that instance of the filter in such circumstances.
A possible workaround is to make sure the frame rate of the project is exactly the same as that of the clips involved."
Cf. http://www.neatvideo.com/bugs.html
In my case no transition is needed to have black clip instead of rendered ones each time Neat Video is involved, on a clean, brand new and up to date Apple config/install. I've never experienced any issue with this plugin before.
So, how comes your not meeting a bug (which is quite frequent with bugs) is of any interest for those who do?