Coaches are programmed to try to get the most out of every practice. There can be no wasted moments in that quest. If one ball is out of play, then the next one must enter immediately, in order that no valuable time is lost.
But in this quest to maximise repetitions and practice opportunities we actually waste a lot of time. When a ball is entered immediately after one is dead, very often the player in the question is still looking at the previous ball, or otherwise resetting themselves. If they are not ready for the next action then whatever it is they do in the attempt to play the ball will be at best an imperfect repetition (because it does not include full cue recognition) and at worst a complete waste of time (because the player sees the ball too late to play it correctly). Either way it is a pointless repetition and a loss of time.
This common error can be seen very often during serve and reception drills in which someone is instructed to serve a ball after an error. It can also be seen during wash drills, when a player/s has to wipe the court before proceeding.
To prevent this error in practice management is very simple. Look first. Every time you want to blow your whistle to initiate the rally, or put the ball into ball, look first to see that every player is ready. When attention is there, put the ball in. The extra second that you take each time will ensure that no rallies are lost and therefore no learning opportunities.
The initial collection of 50 Coaching Tips can be found here.