This is a bug posted on github which apparently affects the git repo: https://github.com/magento/magento2/issues/2989, but it also appears to affect all our composer installations and direct package downloads up to the recent version (2.0.4) from the Magento web site. I haven't seen anyone mention this on the forum here so I take it I'm the only one experiencing this?
Hi - it seems to be an issue with the way files are minimized which we fixed in our internal branch of M2 code we maintain.
For while you could simply get around it by copying the uncompressed custom-options.js into the minimized version after you've made changes to the site and rebuild etc like so:
cat vendor/magento/module-catalog/view/adminhtml/web/js/custom-options.js > pub/static/adminhtml/Magento/backend/en_US/Magento_Catalog/js/custom-options.min.js
I can't really compare notes like I said; we maintain a separate version of M2 for our stores but the above should help until a permanent fix exists in M2.