I am training up my skills. I decided to go train my destruction skills as much as I could. In order to do this I went to talk to Sybille Stentor at the Blue Palace.
I went into her room, woke her up, and trained from level 15 to level 20. No problem. I then leveled up my character and went on about the next set.
I then went to train from level 20 through 25. I clicked to train to 21. My skill advanced and it counted me as having trained once this level.
I clicked it again and my skill advanced to 22 and it changed to reflect that I had trained twice this level.
I clicked it again and my skill advanced to 23, and again the display changed to reflect that I had trained three times this level.
I then clicked it again, to go from level 23 to level 24, and my skill did NOT advance, but the counter showing I had trained four times this level did change so that it felt I had done four trainings this level, even though I only got three skill advances from training.
For curiosity's sake I used my last training for this level to see if I could get my Destruction up to 24 - and it worked as designed. For some reason though, it's not crediting me with training from level 23 to 24, while still using one of my training turns. It only does it once, and if I repeat the process it works fine.
This is kind of messed up. Had I not been carefully watching the screen and tracking my stats, I'd have gotten hosed.
I have not tried it with any other trainers in Destruction, nor have I tried it with any other skills. I don't know what happens in those circumstances. I just know it's happening to me right now with this trainer.