Do you really think they didn't test that before they got to this point?

I'm willing to bet they had been through that intersection before hundreds of times and never seen this. It's not like it can't detect a stoplight and they're just out there randomly running through them all.

Of the millions of variables that were around them something blinded it to light this time. The footage from that run has probably been reviewed at nauseam at this point and is done more for them finding the problem than they could have done sitting in a closed warehouse making guesses when the car never fails to detect a red light.

edit: look keep smacking that downvote, but it's not going to change anything. I hate musk too, but we're going to make progress toward automated driving unless it becomes more dangerous than existing driver. In the next generation or so, most driving will become automated and all deaths by automobiles will drop significantly. Old and young people will get where they need to go. You cannot automate driving without driving in the real world. If you think they haven't been doing this in a simulation for a decade, you're on crack.