On my last few mixes, I've been running into a weird quirk with clipping.
I'll go through the mix in short bursts (listen to a bit; hear something wrong; go back to fix it; continue), and once I'm happy, listen all the way through.
When I'm listening all the way through, the mix will suddenly clip some place that it hadn't before. I'll jump back to a second or two before the clip, and it will disappear, only to reappear when I start playback from the beginning.
Sometimes, the clip will return if I jump several seconds before where I first hear it, but sometimes not.
I suspect this has something to do with compression, but I'm not familiar enough with the math to be sure.