When you browse Instagram and find Tony Abbott's passport number
Nice. Here's a similar personal story with a PSA that sometimes blurring is NOT sufficient.<p>A friend of mine posted on Instagram a picture of a U.S. visa (or something similar; it was probably five years ago) to announce her trip to the U.S., and she took care to blur out sensitive information such as her passport number. But a Gaussian blur is easy to reverse and I successfully unblurred it and told her my discovery. I didn't use any specialized software; it was just Mathematica with its built-in ImageDeconvolve function with guessed parameters for the Gaussian kernel.<p>I personally recommend blacking out (add a black rectangle) instead of blurring, and if it is a PDF, convert to an image afterwards because too many PDF editors use non-destructive operations to add a new object instead of changing what's underneath.