Solobike had a few website issues when they updated there e-commerce plugin. This was because the original developer had made modifications to the plugin code directly as opposed to using hooks and filters to do the overwriting.
Editing the plugin directly appealing as it is easier an quicker for the developer. The problem is these edits will be overwritten when the plugin is eventually required to be updated. It could be 1 week away or a year but eventually an update will be required.
After plenty of debugging and trying to work out why the site malfunctioned we finally narrowed it down the problem and got the site working properly again.