I'm not sure if this one really qualifies, but let's play around with a hypothetical real quick. Let's imagine a world where in Super Bowl XXXI Desmond Howard never scores on his 99 yard kickoff return. It's impossible to know how the game would have played out but the Patriots had clawed their way back and had a lot of momentum going into that play, there's at least a decent chance that they come back to win that game. If that happens Packers history takes on a weird and slightly more frustrating turn. The 90s Packers go on to be seen like the 90s Eagles, instead of being perceived as a powerhouse they're a perennial tease, and Favre goes down as a great Quarterback who just can't win when it counts, and while he still goes to the Hall of Fame his legacy is significantly diminished. Rodgers' Super Bowl run, in the meantime, takes on historic proportions, as he finally manages to take the Packers back to their glory days and likely silences any argument that he's worse than Favre after lifting the Lombardi Trophy. While the Packers are still perceived as a very good team they aren't seen as one of the all time greats like they are now, and for the years before XLV their fanbase is seen as one suffering from a futile search for the year that they'll finally take that next step.

Like I said, I'm not sure if that play is defining, mainly because this is all based on speculation and there's now way to know that Favre wouldn't have just led the offense down the field and stopped the Patriots dreams on that drive anyway(I like to think he would have) but if we remove that play from history there's a good chance the 90s era Packers are seen in a very different light.

So defining or not, every Packer fan should agree on one thing: we should all be very happy Desmond ran that fucker back and put the Patriots two scores behind.