Why does the Vanity Checker give false positives for example with Tumblr?

You have to bear in mind that what the vanity checker does is to check for markers in the page that shows that a page is not live. As a general rule with such properties if a page isn't live it can be registered, and thats the assumption that the vanity checker makes.

However some properties, tumblr for example, will ban a sub-domain/username and when they do that it can't ever be re-registered. So it will show up just like one that is available and you won't know its not available until you try and register it. Since the vanity checker is built for speed and doesn't support authentication etc... it does not attempt to register the properties it only checks to see if they match a given set of markers. So the vanity checker should more be considered a "highly accurate assumption based on probable data" then it should be a "this is 100% available to register".

I hear people about every week say this about tumblr, because it such a popular target, that it has had so many sub-domains registered and then banned and so many people are trying to register subdomains on it that already have links that the vast majority of anything that meets common criteria is already taken or banned. Its kind of like trying to find a google passed proxy, but worse, everyone wants them and so they are few and far between. Same goes for tumblr, so many get banned and so many people are looking for ones to register that what is left is all the ones that are permanently banned. So if it seems like your getting a lot of false positives thats not a fault of the vanity checker, its doing its job correctly in that case, its a fault of high demand and low supply and the nature of the game.

So if the vanity checker is saying its available and you load the page and someone already has it registered, then the vanity checker needs to be "fixed", but if its saying its not available and you load the page in a browser and it shows a 404 or not found/doesn't exist, then the vanity checker is working as it should, making the assumption that since it doesn't exist it should be available, and if you then go to try and register it and its not able to be registered, then its just been permanently banned. In this case there is nothing wrong with the vanity checker and for that matter nothing that could even be fixed, its just the nature of low supply, high demand and the way the game is played.

So then you have 2 choices. Churn thru massive amounts of them to try and find a few you can register. Pick a different platform thats not so heavily used, or build in your own platform. (I have a video on this - https://www.youtube.com/watch?v=-F2nr_ltCRo )