but when user gets that "Cannot rename ..." message when dropping indexes and FKs it is hard to get the real cause of the error unless he executes
show engine innodb status
.The problem is that resulting table is not accessible directly, because if you double click on "Status" you get "Could not determine name of table" error.
The only way is to copy the contents and paste it in an editor. It would be nice to provide some UI access to that info. Especially to "Latest foreign key error" section.