When you have a directly full of numbered images, the images are not ordered correctly.
Eg: 100.jpg, 101.jpg, 102.jpg, 103.jpg, ..., 10.jpg, 110.jpg
They should have been ordered like this:
1.jpg, 2.jpg, ..., 99.jpg, 100.jpg, 101.jpg
A workaround is renaming the files to include zeroes (001.jpg, 015.jpg) but it's not very convenient to do that for 99 files!