Problem: Was trying to upload a custom wordpress theme from themeforest.net and during the upload the theme uploader was crashing out prompting with ‘Are you sure you wan to do this? Please try again.’

Solution: Easy one really, ensure you are uploading the WordPress theme only and no other (the zip file will typically be the name of the theme.

The zip file will typically be:

themename.zip > themename.themeversion > with css, img, inc, js languages and plugins folders and a bunch of php files.

Good luck!