When you create a traning instance and you re-assing the pool during the traning some users can no longer access it.

I know it's an odd scenerio but it happened twice for us. Once by mistake we had a local pool and then when assigned a correct pool it's been working for 99% of users but for some failed with the message "You already have a sandbox assigned. Use that one or ask your tutor for help." On another occasion, support staff, by mistake, assigned a very similar but wrong pool to the training instance, and after assigning the correct pool again, a couple of users couldn't continue the training with the same message.