It seems that the factory reset for Android 4.3 and below is flawed, in that researchers were able to recover data from wiped phones.  Two University of Cambridge scientists tested 21 phones from Samsung, HTC, Nexus and 2 other unspecifed vendors all running versions of Android ranging from 2.3 to 4.3 and were able to recover data from a supposedly wiped phone.  They did not test newer versions and so are unsure if the problem has been rectified nor did Google respond to The Register when they inquired.  The researchers had a success rate of 80% for recovering tokens for Google and Facebook and could even recover encryption keys, although the keys were still password protected they could be brute forced.  Make sure to encrypt your phone with a long password before you wipe it and sell it, give it away or toss it out!

"Half a billion Android phones could have data recovered and Google accounts compromised thanks to flaws in the default wiping feature, University of Cambridge scientists Laurent Simon and Ross Anderson have claimed."

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