Consider a board with several installed languages (at least two). The administrator has to define a default language for the board (for the example, say it's 'British English').
In that board, the administrator also enables the avatar gallery (disabled by default) and all avatars are located in the root of avatar gallery path (such as /images/avatars/gallery) so that there is no avatar category.
Regardless the user preferences, the dropdown for avatar category displays the same word ('None' if English is the default language). The problem is the following : if you use an alternative language (not the board default language defined by admin), the avatar gallery feature simply doesn't work. If you select an avatar, the link associated to your profile becomes incorrect (/images/avatars/gallery/None/avatar_chosen.jpg instead of /images/avatars/gallery/avatar_chosen.jpg), resulting an empty avatar.
If the admin changes the default language (and then purges the cache), the bug will be reproduced for others language (except the new default one).