Hoping you can help me with this issue which has recently started happening. I'm editing like normal and letting it render as I go. Everything seems fine but when I playback I have these random pixelated frames where it's grabbing a copy of another frame(s). It seems the more I fix them by replace the clips snd the longer I'm in the program the more it happens.
I've compounded the clips as I thought that would help but it even happens to frames within compounded clips. I've got deleted all my render files and that seems to bring in new ones.
I've run into this problem as well, including on a project I'm currently working on. Unfortunately, I don't have a guaranteed fix, but I've found occasionally if I Delete all my Render Files it will help. Looks to me like it's a rendering error but that doesn't fix it every time.
Give that a try and see if it helps! (I'm on Mojave, 10.14.6, FCPX 10.4.6 as reference)
As a follow up, I literally just had this happen. It was pixelating and crossing frames with a different clip from earlier in the project. I deleted the render files and it seems to have worked in this case.