This should be safe if done briefly, right?
When it turns back into a table, we get bubbles in the table and sawdust in the air.
If a large amount of transfigured air has been incorporated into the bodies of bystanders (and pigs do breathe out oxygen, or at least humans do, because our lungs aren't terribly efficient), then we might have a problem.
That doesn't seem like a sufficient degree of caution, considering how McGonagall behaves with respect to other potentially dangerous Transfigurations. She doesn't say to only Transfigure things briefly into liquids because of evaporation, she says not to do it at all.
And she especially wouldn't do a potentially dangerous Transfiguration as the introduction to a Transfiguration safety lecture without making a big deal out of all the precautions she had to take to do it. In fact, she specifically says that the students could guess the direction of the Transfiguration based on safety principles, and based on safety principles, it should have been pig to desk, not desk to pig.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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