Quite a simple question. Maybe space for confessions (a little Catholic perhaps - relinquish your guilt).
How often in a training plan do you fail your workouts? I don’t mean ‘just about hold on with a 2% reduction’ - that’s still mostly a success, but a proper failure.
And then what did you do in response?
Rarely and usually nothing - had a flu bug 10 days ago, came back did Avalanche spire, was finding it tougher than expected so binned the last interval. Still go a decent workout and had a reason so just moved on. If you start failing more often then a bit more investigation is needed. 
In my opinion, if you’re never failing workouts, you aren’t getting the maximum out of your training. I would argue that a massive part of cycling is recognising how much pain you can truly endure if you have to. Realising just how long you can hold on is exceptionally valuable for both training and racing. Ask yourself at the end of an interval - is this really all I can do, do I HAVE TO quit? If someone gave you 1,000 dollars for every minute you held on, could you hold on for longer?
I’m not saying that every workout or interval should be maximally taxing. Sometimes, 4x15 minutes at threshold feels quite easy, and that’s good. However, extending an interval can be very insightful. Just how long can you hold on if you have to? Our minds tend to fail us long before our legs do. Therefore, once in a while, I would argue that extending an interval until you simply cannot physically pedal anymore is valuable. Anyone would be surprised by how capable they truly are. The next time you do threshold work, just don’t stop when the interval is over. Keep going. Keep pushing until you literally fall over. That’s the limit, and if you never get there, you never realise how much strength you leave in the tank every workout.
I think this is one of the major differences between amateurs and professionals. Pros are so much better at extracting everything out of themselves when it’s needed. You don’t need to do it often, but in some workouts, and in all races, you need to find your true limit, or you’re leaving a lot on the table.
In terms of “how often”, I don’t know. I almost never fail the prescribed workout, but about once or twice a month I’ll extend the last interval until I cannot pedal anymore. At that point, I literally can’t stay upright on my rollers. I get off, lay down on the floor, and take note of how far I managed to push myself. I think a majority of athletes would benefit from finding failure a bit more often.
What is considered a failed workout?
Completing 4-4.5 of the 5 sets at VO2max, i.e. giving up the last one.
Perhaps being unable to hold a certain power level right from the start of the first interval?
Reducing the bias to 90% of the target power? For those that ride with ERG on.
I’d say completing less than 95% of the TSS of a workout, or skipping any set of intervals.
3-4 times a year. Usually centered around illness or FTP being set inaccurately.
First one last year I got C19 the next day and failed a threshold workout.
Next time failed one after a death in the family and just had too much anxiety/stress
Next time threshold was set 15 watts too high from AiFTP
Last one was threshold workout after flu + norovirus so three weeks off.
I usually don’t do crazy workouts outside of my own personal time in zone progressions so if o just did 2x20 I can usually do 2x25 the next time (albeit with difficulty)
Pretty much never so far, with maybe one or two exceptions, basically not sleeping the night before and trying to do a workout I had no business trying. I’ve been on a LV plan so what I do if that happens is get off the bike, move the workout to the next day, and I’ve always crushed it when rested and ready. Most workouts are Hard, Moderate, Easy very rarely have I had a “Very Hard”. I think this is pretty much the goal of adaptive training though - “Minimal Effective Dose” and steady gains without killing yourself. Consistency over potentially overdoing it.
Honestly, agree with the above, makes me feel I may need to push myself harder in certain cases and really learn where my limits are better. After a year of TR Adaptive Training, Working with a coach now for the first time going into a VO2 block, so ask me if that’s changed in a month or two.
Touchwood, I cant recall when I’ve properly failed a workout. I like to train in the TT position when I go out that to fix on and adhere to the target I judge that as a semi failure; I can recall doing that but fortunately not often.
I’ve returned to Resistance mode training and although I haven’t failed a work out yet, eventually I’ll reach a point when I cant meet the target but I’ll fail hopefully without injuring my self whereas I’ve found with ERG I am too stubborn and have injured myself.
I dont fail a workout that often anymore. I am better at recognizing when work stress and poor sleep will impact me. I have learned when to not push myself which means I would typically tone down the planned workout via an alternate or just do the workout at 98%. At my age there is no rush to get anywhere it is more a function of ensuring a slow steady progress.
When I first started to train I would get down to 90%…it really just meant FTP was too high.
These days rarely touch it but if I do it is to just make the ride slightly easier at say 98% vs finding an alternate workout.
3 or 4 times a year. Usually when I knew better, but tried anyway.
That’s not a failed workout, in my opinion, unless it happens often. Then, as you say, FTP is too high. Some days you mail it, other days not.
95% is a bit tight. TrainingPeaks marks a workout green when it’s less then +/-20% of the planned value.
Since adaptive training, almost never.
If I stop a workout it usually means I need more rest and less beer or illness.
If your FTP is 250, 5% is taking 12 watts off a Threshold interval. I think that’s a lot.
The threshold zone is 90-105% so that’s a big swing between feeling good and doing 100-105% vs feeling bad and doing 95-105%.
It’s still in the range, so still targeting the same energy system.
Not if the target was 90%
If your FTP is 250, and you fail at 90%, then is your FTP really 250? It depends on the duration of each interval.
There are lots of factors at play, but, IMO, it’s still a fail if I had a target and didn’t meet it by more than 5%.
That’s why I asked, “what is considered a failed workout?” Opinions would differ based on the goal for that workout.