Sadly, it's a word that leaders often use, almost automatically, without fully appreciating its effect. A headteacher friend tells me that once, relaxed in the pub after a parents' evening, the deputy was emboldened to say, "You know, whenever you come round and praise some work - a wall display, a class assembly - we know that eventually you're going to say, 'But...' We call it 'waiting for the but'."
You can see what's going on, can't you? The leader who does this doesn't think of it as negative or critical. On the contrary, it's seen as positive and developmental - in effect, "You're doing well. Now the next step for you is to do this and this..."
But all the person on the receiving end hears is the dead, punctured-tyre sound of that little word.
I suggest there are two things that leaders can bear in mind when they feel the word "but" coming on. One - and newly promoted heads are often slow to realise this - is that your colleagues take even the most casual of your utterances with terrifying seriousness, so no matter how mildly you deliver your "but", it's always received in capital letters.
Then there is a valuable piece of general advice, which I recall from some guru or other, that says, "Never tell anyone they're wrong. If their error is a small one, it doesn't matter. If it's a big mistake, they'll find out soon enough."
For a leader intent on raising standards, that's a hard one. What it means, I suppose, is that when you are praising something that is well-meant, excellent in general terms but flawed in some obvious details, ("Super artwork in the corridor, Jean. I'm really pleased.") then even though you know how it could be improved, you should swallow the doubts and leave things for a while.
After all, Jean may well pick up some tips just by talking things through with her colleagues.
In any case, you've now learnt something about Jean's developmental needs, and as time goes on there will be ways of addressing them that don't involve spoiling her big moment.
But the point is, try not to say "but". OK?