While Live is busy with Copy or Paste operation of memory intensive tracks (try Kontakt with "Concert Grand" for example) the keyboard controls don't work inside Live anymore until some time after the operation is completed. That means you cannot use cursor-keys, del, space and neither ctrl-v (Paste on Windows).
The mouse on the other hand is still perfectly useable, including chosing Paste operations while the keys are still disabled (that is why I wrote "some time after").
I reported problems with that as early as last January but obviously no one at Ableton took a closer look at it since then. Once you know about the problem you can workaround it, but it can be very confusing in times when you try to work fast.
Keyboard dead while using Copy or Paste with large tracks
Keyboard dead while using Copy or Paste with large tracks
HAL: I know I've made some very poor decisions recently, but I can give you my complete assurance that my work will be back to normal. I've still got the greatest enthusiasm and confidence in the mission. And I want to help you.