Never make hollow ko threats
Never make a ko threat that you won't follow up on if your opponent wins the ko.
Jono: This "proverb" was inspired while watching weak kyu players make a ko threat and then proceed to ignore it after losing the ko. Basically they managed two threats and capitalised on neither.
Alex: There are two forms of this, only one of which is preventable.
[1] 1) Making a ko threat that doesn't work, because you misread it. This isn't really preventable - all you can do is try to read more carefully, and get better through experience, or hope that your opponent doesn't realise that it doesn't work either. I've even read cases of pros playing what looks like an obvious threat impulsively, without realising that the opponent can ignore it and defend skilfully.
[2] 2) When the ko leads to a severe follow-up (for instance, if the ko itself is only to capture a few stones, but the defender must answer after his opponent wins the ko or else lose his whole group), your threats don't just have to be bigger than the ko, they have to be bigger than the ko plus its follow-up, or else you won't get to follow through on them. This mistake comes from not recognising what is really going on, but once you've been told to check the follow-up for a ko before fighting it, it isn't (usually) so hard to do so. This kind of mistake is preventable, and disappears (for the most part) at a certain level, though I'm not sure offhand what level that is.
See mukou.
tderz: It would be difficult for me to follow this advice or proverb literally and always.
What is the teaching of it? - "Do not make mistakes ...!" has not much concrete didactic value. [1] clarified this.
Alex's [2] elaborated on the evaluation as seen from the "defender's" view.
It could be added that the ko-"attacker"" usually thinks in terms of the "value of 2 moves" in succession (for direct kos).
[3]) In cases where the value of 2 moves,
being the "ko threat" + its local continuation - hereby called a1 + a2 - is smaller than the sum of the same a1 + another b1 ,
than above advice is simply not true.
These situations could easily appear in the (late) endgame, when several big moves are left, but groups are not killable with 2 moves.
Above advice will usually hold (be true, i.e. s.o. made a mistake) in any stage before (e.g. middlegame).
That "normal moves" (without a special or big continuation) can be ko threats (if you have nothing better) is also explained somewhere (I do not take the time now to retrieve the exact ref.) in Charles (in collaboration with Seong-june Kim) excellent http://gobase.org/studying/articles/matthews/ko/ articles on ko.
Alex: True enough. Ko is more complicated than most amateurs appreciate. As another example, you often see, in professional games more than amateur, a ko fight start, but then the result of a certain ko threat increases the value of other areas of the board enough that both players take a break from the ko for a while to play normal moves elsewhere before returning to fight the ko again. It's a matter of semantics whether you want to think of this as abandoning the ko for a while because other matters have become more urgent, or as a sort of whole board ko threat.