BTW, didn't mean to be overly shouty with the caps lock, sorry if it came off like that ^^
It is just a terribly bad thing to do and most of the people who do it are those who are at the highest risk, i.e. newbys. Done it myself plenty times and see others doing it all the time.
Even now I still get tempted, yesterday I missed my 4x5 min session on the bike, missing the session was my fault and I felt guilty, so today I was very tempted to double up and 4x5 min it on the morning then a 40 mile aerobic endurance ride this evening. Knowing full well tomorrow is pencilled in to be a 2x20 threshold session. I would be smashed by the end of tomorrow if I did this and the threshold session probably would not be very productive.
In such circumstances you need to think, which is the more crucial session at this stage in your season, for me in this example, the choice is do I sacrifice a 40 mile endurance ride with the club or a high intensity ride? In my case my aerobic base is pretty good (less than 3% decoupling between power and HR over a 2 hour ride), so by skipping this endurance ride I am not sacrificing much other than a bit of a social and can fit in my high intensity session today and recover enough to perform my threshold session at the right intensity tomorrow.
It is hard sometimes but you need to think like this to get the best out of yourself.