Someone at OOo decided that context sensitivity of toolbars was a good idea. I disagree (and have lodged bug reports), but the OOo people prefer to agree to disagree on this issue. What happens with context senstivity is that when OOo senses you are doing something which might need a specific toolbar, it pops it up for you. Equally, if it thinks you aren’t it will remove that toolbar for you (how kind).
So imagine this situation: you have a document which includes some numbering. You’d like to add numbering to a currently unnumbered paragraph. So, you click on the paragraph and then go up to the numbering toolbar. In the split second it’s taken you to click and move, OOo has realised you’ve just clicked on an unnumbered paragraph – so you mustn’t want the numbering any more. Just as you’re about to click the button on the numbering toolbar to add numbering, ‘poof!’ it disappears.
rest here