Not even close to being the same thing.

A military exercise gone wrong is indeed a tragic accident.

However, a plane being shot down intentionally (the only accident being it was the wrong plane) by pro-Russian separatists who are waging a proxy war funded by Putin's Kremlin, due to the annexation of Crimea months prior, all able to occur by the belligerents taking advantage of the currently weak Western powers, and an incredibly inactive United States, is by no means an accident that holds little significance. Rather, it holds a lot of significance. If Putin doesn't back down, and it looks like he won't, then the international community needs to get involved (as they should have done with Crimea in the first place).

Right now we're entering a Cold War mentality.

But if Putin continues this behavior, either directly or indirectly, and the west continues to appease and acquiesce to him, then we're marching very dangerously towards the circumstances that led to WWII where Hitler's annexation of the Sudetenland eventually escalated to an international problem, as Hitler continued his aggression and invaded Poland, and thus WWII. Obama is very much like Neville Chamberlain in this regard, he thinks negotiations and rhetoric will indeed sufficiently appease any adversary, and allow "Peace in our time." Chamberlain was wrong, and so is Obama. More needs to be done now, before things can get worse - which may very well, but significantly does not have to, include WWIII as a consequence.

And, further, there are elements of WWI at play, too, with how quickly a regional event spilled over.

WWIII may seem far fetched, and to some degree, it is. But add Israel, Hamas, Iran, et al to the list, AS WELL AS the crisis in Iraq, and the very unstable situations in both Libya and Syria, and you have all the key players and alliances in place. There's a loose thread that ignites all of them, let's hope that no one lights the consequential match.

And even if this all plays out on Cold War minded terms, which is the most likely out come, it's a depressing outlook. For one, the Cold War remained chilled because of strong, or equally balanced at the very least, leaders who had mutual respect and fear for their adversarial counterparts. Putin doesn't see Obama as strong, he sees Obama as weak. And that's exactly a huge factor in why the world is in utter chaos right now, and why US - Russian relations aren't in our favor, nor do they hang in the balance, but rather tilt towards the former Soviet state. Unless Obama finally shows some strength and leadership, all of this bodes very badly.

So no, this is nothing like an unfortunate military exercise gone wrong.