if you have checked that the variables really are changing, there's only one I can think of: If you press Ctrl+Sh+S, all objects on a sheet become visible - which is quite practical in a way - but that overrides any visibility_conditions.
You have to actively reset that by pressing the same three buttons again.
glad we hit the right one.
Well, yes, it seems to be hard to do by accident - but then, how often do you change some setting in your IE or something and you have no idea how you did it, whether it's cool or not ... Happens quite often to me. At times I think I need a keylogger to be able to find out what I did when I have hit upon some cool setting that I wish I'd be able to invoke at will ... at other times it's not so funny when text becomes unreadably tiny and stuff ...