I am getting a "Out of Memory" error. I have the same clip overlayed on top of another, and have the top clip cropped. The bottom clip is at a lower opacity. When I try to render that section, it gives me a "Out of Memory" error message.
Do you have space left on your scratch disk - i.e. where your render files are being written to?
Other reasons this happens is if you have weird resolution photos in your timeline, or your computer is actually short on memory and it's time to upgrade that RAM. You can try shutting all programs, reopening just Final Cut Pro, selecting just those clips, pressing (CMD + R) to render selection.