This is an interesting one that got us scratching our heads yesterday. It also looks like we're not the only people who have noticed this strange bug in Final Cut Pro X.
Maybe it is because we come from an analogue editing background many years ago when tapes used to be rejected for bad blanking errors, but we watch the edge of video frames like hawks.
So image our surprise when we saw that pixels were being lost on either side of the frame when a transition was applied. This doesn't happen with all effects in FCPX, just some of the ones that are written in Motion5. We saw the problem when testing out a new transition with 720x576 anamorphic footage, it doesn't appear when using media that has square pixels. Other people have noticed the FCPX bug too.
It is particularly noticeable as the outgoing video stays behind the incoming video until the end of the transition. That results in an edge and the incoming video popping back to its original size.
We thought it was an error in some of the coordinates and maths in the custom transition, not so as you can reproduce the problem with one of Apple's built in effects.
Take a look at a split screen between two anamorphic timelines, the top half is during the transition and the bottom half is after the transition. The transition used is 'Squares' which can be found in the object listing in transitions.
You can clearly see that pixels are missing off the right and left hand sides. Using fit or passing the footage through Compressor to add the anamorphic flag gives the same result.
We are running FCPX 10.0.1, Motion 5.0.1 and the latest version of Snow Leopard. We don't know if this bug was present in the first version of FCPX, but we do hope it'll be fixed in the next update.
The best way to report bugs to Apple is to use their feedback page.