Originally Posted by u-Bob

Hitler's mother wasn't jewish. His grandmother however did work as a serveant for at least 2 jewish families... she got pregnant and moved back to her home town and married a guy named Hiedler. She named her son, Alois Hitler. Adolf Hitler was the son of Alois Hitler and Klara Pölzl (a churchgoing Roman Catholic).

So there is a possibility that Adolf Hitler's biological grandfather was jewish. There are several theories about this,
possible grandfathers:
a man called Frankenberger (jew)
baron von rotschild (jew)
Hielder's brother (not jewish)
...

There's also a theory/rumor that Engelbert Dollfuss (Chancellor of Austria) had a file about Hitler's ancestry (a kind of insurance policy) and that this file ended up in the hands of Reinhard Heydrich after Dollfuss was murdered by the nazi's.