In the 8 years I worked as a tower controller in one of the busiest airports in the canary islands, I had three airliners taking off without being cleared to, they were just cleared to line up and wait (one german, two spanish).
I also once forgot to clear to land an aircraft (my mistake) but he landed anyway. And I could continue on and on...
I don't intend to justify what happened, it certainly sounds like a serious situation, but there are many factors to be taking into consideration: if the controller was getting his twr rating, workload at the moment, visibility... The important thing is that he realized his mistake and took immediate action.
And we must accept that mistakes happen everyday in both sides (I'm no exception, but my mistakes are
).