Didn't see the match because I was on the road - but it sounds like Steyn made two bad kicking mistakes. Didn't Crusaders do something like that a few weeks ago against the Sharks, not playing smart after the hooter?
Anyway, point is that players do it so often that you'd have to wonder if the situation on the pitch is as clear to the players as it seems from the stands, or to the TV audience. Exhaustion after 100+ minutes of rugby intensity makes you do strange things.
There was an example from the Guinness Premiership a couple of months ago (I think) where an outhalf kicked the ball from his own 22 across his own goal-line a few minutes before the hooter, thinking that would end the match - it resulted in a restart. Maybe they just don't hear, or don't have time to check.
In the 6N, Wales kicked a penalty to touch against Italy with 10 secs to go, but the lineout wasn't taken before the hooter - game over. And Ireland tried to keep the phases going against Italy after the hooter to run up more points, but Italy took possession and notched a score that arguably gave the championship to France.
My favourite match-finisher is Peter Stringer, Munster's #9 - always alert to the ticking clock, and when his team has the lead and full time passes he ALWAYS lashes the ball into the stands. It's a trademark for him. But then he didn't do it in Ireland's 6N match against Italy. Doh!
It's so confusing - unless you're in the stands.