Yeah, I had my suspicions that in not defining a tailing loop, only its cause, I would create an argument on the board! I suppose that it depends on how you choose to look at flycasting. I consider all presentation casts (slack line and curved casts) to be intentional bumcasts. Casting faults can only be relative to our intentions. It's pretty easy to find uses for tailing loops - a vertical tailing loop throws a great tuck cast for example. Start altering the plane of tailing loop and the cast stops looking like a tailing loop, but since you've travelled along that path, you know that it's still a tailing loop.
I've talked about Bill Gammel's 5 Essential Elements before and how they are quite brilliant (no one's sure how Bill managed to come up with them) and they are: the  Straight Line Path of the rod tip, which is both an action and a result of:  proper casting arc,  proper power application,  proper timing and  elimination of slack line.
OK I'm not trying to teach you to be an instructor, but if you combine Bill's 5 Essentials with Bruce Richards' Six Step Approach…  what's the loop fault?  what was the tip path error?  what was the body/hand movement?  fix the hand  straighten the tip path  Sexyloops. (actually Bruce didn't say “Sexyloops”, I just made that bit up)… then you have the most incredible casting analysis tool available. It really is quite amazing.
Anyway I'd like to fuck with it now.
If you reverse everything you can take  a Sexyloop, work out that's a result of  the Straight Line tip Path,  analyse your hand movement and then  intentionally disregard one (or more) of Bill's 5 excellent Essentials,  analyse the tip path and  admire your new casting creation.
I drop the description of “tailing loops” into position  and use the term tailing loop to describe a tip path where the tip dips under the Straight Line Path [in the plane of the rod]. Of course you could define tailing loops in position  which most instructors do, but it becomes almost meaningless… if I had to define a tailing loop in terms of position  (you know like if my current lifetime depended on it) then I would say "a tailing loop is one where the top leg of the loop has dipped under the straight line path [in the plane of the rod] before the Straight Rod Position."
Of course if you were to watch this loop unfurl this dip appears in the loop. However I'm being very careful to define tailing loops in terms of tip path because then the analysis actually means something and it becomes useful from a practical casting position.
Oh by the way, if you are reading this page because you have a tailing loop problem then you have come to the wrong place: this is the page you should have been reading.
Let's have some art:
In this great diagram you have an approximate tip path for a normal overhead vertical loop. It's never perfectly straight and the tip travels a slightly convex path, especially at the beginning of its path.
This is the tip path for “forward creep”:
And here it is in full cataclysmic action:
A useful presentation cast that one, I'm sure you'll agree.