The first is that the push back and start went extremely quickly and the aircraft arrives at the hold 5 minutes before its CTOT margin (i.e. CTOT minus 10). All aircraft behind it in the hold get held up by delaying that 1 aircraft, and it's possible that another aircraft may miss its slot as a result.
Any ATCO who manages to do that wants their a$$ kicked. You either hold the push and push in slot order, or you remote hold and get the earlier traffic past them that way. So - a competent ATCO won't let that happen and because of that the aircraft
WILL NOT go outside slot.
The other scenario is that an aircraft arrives at the hold just before CTOT plus 10 minutes, but a landing aircraft reports debris on the runway meaning that a runway inspection is required, or perhaps there is just a lot of congestion at the hold. Is it really sensible to make the aircraft that's missed its slot taxy back to stand and refuel rather than let it go at CTOT plus 12?
In such a scenario you would get a slot extension from the Flow Desk - you would not "shoot them off" without reference to them.
As for congestion at the hold - see answer 1.
It's important to reiterate here for those that can't grasp the concept (and has been mentioned here before). The CTOT is actually an absolute as far as you the pilot is concerned - you should be working to be at the threshold at that time. The -5/+10 minutes belongs to ME - not the aircraft