I just launched a project, and despite using a heavily tried-and-tested script for dealing with serialized data in migrating, we ended up with some custom field values that weren’t being output (in admin or on the front-end) because the serialized values had become corrupted. I think it must have been something strange to do with characters pasted in from Word, maybe in conjunction with the recent WP core upgrade to handling utf8mb4
data. I’m not sure.
In any case, I found a PHP script that repairs serialized data, and turned it into a plugin: Repair Meta.