Can someone tell me the upload limitations? no idea if the severs are on a slow day or if the picture is too big, i keep a 520 error when i try to upload so i am going though possible issues.

520 means e621 can't be reached. I don't think it's related to upload limits.

Very rarely, I would get some sort of error with some very specific picture that I want to upload, I tested the tags, the filename, everything : It just wouldn't want to be uploaded. The file size wasn't big too, so the reason remained a mystery. You may have the same problem that I had, but best way to know would be to show the picture you're trying to upload

One of the app servers dropped offline due to a power failure and was out for an hour or two before it was removed from the rotation. Are you still getting these errors now that the problem has been fixed?