What happened at Qana is a tragedy. But it isn't enough to say so, nor to say that it was a 'mistake'. And it isn't enough, either, to insist that Hizbollah bears responsibility for the deaths of civilians from Israeli bombing, because of the way that organization uses the civilian population to shield its military resources and activities. It does bear that responsibility - and those currently lauding it as a resistance movement have crossed a significant moral and political threshold (of which more later). But it also isn't enough to point this out.
Israel's action at Qana is inexcusable. I say this as someone who supports Israel's right to defend itself against those who attack it, those who send missiles against its civilians and who intend its destruction as a state. But if Hizbollah bears the responsibility identified above, Israel also has a clear responsibility under the laws of war to try to minimize civilian casualties; and this applies even in the case where its enemy resorts to the use of human shields (as explained here by Michael Walzer). I don't believe the position of the IDF - that civilians had been warned to evacuate the area - adequately acquits it of that responsibility. It is not a reasonable expectation, especially not in conditions of war, that all civilians living in an area will move away in good time, or be able to do so, when warned by one of the warring parties.