The bug is that if there is only one button, then when it vanishes or if the last time you were in the view it was hidden, the action button bar also goes, and then the button and the bar do not return.
A bar that vanishes and never returns how annoying!
Solution:
Add an Action (Create->Action->Action ) in view, and change it's type to Menu Separator, and don't put any hide formulas on it.
The button bar will then have to hang around showing only the menu separator even when the only action button is hidden. Crafty.
NB To do this document context sensitive action behaviour you need to tick
" Evaluate Actions for every document change " in the view properties on the i tab.
NB2: You can have some nice context sensitive buttons based on view columns by using @GetViewInfo in the hide formula.
This is how the Mail Template's Calendar application buttons work I think.