I used the wrong word, sorry.
Please give me a second chance to clarify what I was trying to say 
Pin signal triggers an interrupt => we are inside the "pin ISR" and we call delay() to debounce (wrong way to do it, as already discussed, but this is not the point now). Timer0 interrupt fires at some point, but it doesn't get serviced because of global interrupt disable.
If the ISR just took too much time to execute, at some point it would terminate and the Timer0 interrupt would be eventually serviced, perhaps just a bit late.
If we call delay(), though, we are waiting for the time counter to advance. But that counter is advanced by timer0 ISR, which as we saw already is not serviced. So we wait forever..., the "pin ISR" never exits and everything grinds to a halt.
Now to see whether my reasoning is correct I should (re)read wiring.c very carefully and try some code