After Effects CS3
OS 10.6.3
I have never had this happen with PNGs. In fact I have made 3 other videos with a PNG sequence in it. Each time I make a new PNG sequence, I replace the one that was already there in my project. I have done nothing different. The only thing I’m thinking that it could possibly be is that I recently moved my files to another drive, so that I could partition the ones that they were on. I deleted the files that were in the project that were missing due to the partition, since I didn’t need them anyways. But for some reason AE has seemed a little buggy ever since.