Does anyone know what could be causing this? Get a burst of pixels on the first frame of a clip on the timeline now and again. It's in the exported file as well. Any ideas?

Jon, what Mac are you using? Mac Pro 2013?

Happens to me from time to time when viewing certain kinds of footage like AVCHD and XAVC. Not sure why.

Jon, what Mac are you using? Mac Pro 2013?

Late 2013 iMac sir. I'm currently looking at a clean install of OS as it's overdue and see if that cures things. It just seems to happen quite randomly so there must be something buggy in the system.

Happens to me from time to time when viewing certain kinds of footage like AVCHD and XAVC. Not sure why.

Yeah it's XAVC footage. Serious pain.

I've seen this at times editing 4:2:2 10 bit XAVC-L clips natively in FCPX. It seems to happen more often when I've applied a transition or something and tried to play it before it renders. It's like the drive can't read the source fast enough and it corrupts one frame of the clip. And, I have the current fastest iMac with 4GB GPU with a RAID 0 as media drive.

I've never seen it in source footage though, and can usually fix it by just replacing the clip in the timeline with the source clip.

Is this the first frame of the source clip or the first frame of the trim on your timeline? Is it consistent with the same clips every time or does it crop up randomly every time you export? Usually blocks like this are from an compression decoding error, as if the key frame isn't there or isn't being read.

It does seem to crop up on the first frame of the same clip. Do you reckon transcoding to pro res might help things along? Will give it a shot I reckon.

None of my camera footage has ever had this problem but I have received files that do the same error on the very first frame of the clip. I wish I could tell you what camera those files were from. I also received footage from a camera that was clearly malfunctioning and the files had random blocky patches from an encoding error.

If you trim two frames in, does the error go away?

ProRes might help.

Encoded the XAVC to pro res and all was well. Just something about the native footage that was causing a problem. It is possible to trim the frame out and it's OK but I'd rather take the hit on storage space and not waste time messing around with frames in the edit.