Actually here is the reason I got from technical support:

"Hi Todd,

I was wrong, it didn't turn out to be an interruption during upload. We
found a issue internal to the script which displays uploaded images. It
has to do with an occasional connection interruption in the process of
getting the image from AmazonS3, caching it locally, and then
distributing it.

I'm not sure of the exact specifics, but I'll update this support
request when it's resolved.

Cheers,

Jeff
Sparklit Support"