This is very unclear, in fact my husband and I had a long debate about this rule just yesterday. My take is that you start to move through the enemy square, which triggers an AOO. You use your keen acrobatics to avoid the attack. If your acrobatics are not so keen, then the attack takes place, stopping you before you can move fully through. Basically..a failed tumble means AOO and being stopped just adjacent to the foe, no moving through. That's how I run it at my table. My husband on the other hand runs his table that a failed tumble is simply a no move through, no AOO. It is his opinion that an acrobatics check eliminates the risk of AOO either way. What we do agree on is it is a matter of GM discretion and not defined in the rules one way or another.