PUC already used `upgrader_process_complete` to remove hooks when the plugin version it was part of was deleted during an update. However, that did not catch more obscure situations, such as apparently being called from an unrelated AJAX request while the host plugin version was being deleted (a user sent a stack trace where it seems that was what happened).
When a plugin update overwrites PUC with a different version of PUC, the hook callbacks registered by the old version can trigger fatal errors when they try to autoload now-deleted PHP files. Normally, PUC avoids this by using an `upgrader_process_complete` hook to check if one of its files still exists, and removing the hooks if the file is missing.
However, it appears that WP Last Modified Info has its own `upgrader_process_complete` callback that runs earlier. That callback tries to download plugin metadata, which indirectly triggers some PUC hooks, and leads to the fatal error(s) mentioned earlier.
Fixed by extracting the relevant part of `upgraderProcessComplete` to a separate method and registering that method as a callback for the same hook, but with an earlier priority (1 instead of 11). It appears that WP Last Modified Info uses the default priority: 10.
Yahnis Elsts