First of all, the "pastness" of looking out into space doesn't really affect probability of witnessing them.
Second, supernova occur quite frequently: http://en.wikipedia.org/wiki/List_of_supernovae That's not even a complete list, just the "important" ones. The odds of one occurring within our lifetime that is naked-eye visible are really quite good. The odds of it being spectacular much less so. It's likely to just be a point in the sky that wasn't there last week and won't be there next week.
Second, supernova occur quite frequently: http://en.wikipedia.org/wiki/List_of_supernovae That's not even a complete list, just the "important" ones. The odds of one occurring within our lifetime that is naked-eye visible are really quite good. The odds of it being spectacular much less so. It's likely to just be a point in the sky that wasn't there last week and won't be there next week.