During the Holiday sales, I logged in to Steam on a different computer (one powerful enough to actually run my games :). When I logged back in on my notebook, all of my categories except one were deleted, and all of the games except one were removed from my favourites and from the one remaining category. I re-added some of them.
Later, after logging in again, I lost all my groups and favourites again. Is this behaviour common? Can I get them back, or at least back them up in the future?
edit: yesterday's patch (7. 10. 2015) seems to address this bug (release notes here):
Fixed bug causing category info to be missing from the Library in Offline Mode, and sometimes cleared permanently when going back online
Took at least 3 years, it seems.
(I have read this, but I'm not sure it's the exact same problem. Both my computers ran Windows and I lost my categorization entirely.)