Why can't I see some pictures on Facebook that I posted?

I don’t know what causes this, but sometimes Facebook has trouble loading certain images – even if it’s the same size as those that precede and follow it, a photo might not come up.

If you click Escape while looking at photos, the photo viewer reverts to the old style (without the lightbox). Sometimes, that will allow a hard-to-load photo to be viewed, but sometimes not.