While Eric is talking about factual errors on his blog (orthograph, mistakes like Christopher Colombus watching Indies through a not-so-soon-created spy-glass), it gives me the idea to write something about errors, freaks and oddities (EFO ; in French : les variétés). Something that happens accidentely during the printing process : red instead of blue, the disparition of a colour because of missing ink, no perforation or not the perforation normally used, etc.
Large subject when you read this discussion between French collectors in december 2006 : how did these EFOs manage to get out ? Tired controllers ? Fraud since their value can be important on the market ? Some are falsifications, using professional chimic means and restoration techniques. And of course, those who are blamed in the end : the stamp dealers and the experts who signed the EFOs when a costlier certificate may be better on the long-term to proof the EFOs is genuine.
Back to pleasure since we are philatelists for that : what can you do ?
First idea : examine those presented in magazines and by dealers, then you study how they could have appeared. Excellent training to learn the postage stamp printing process.
Alternative (quite hazardous) : go without thinking to the post office and buy for your mail some definitive stamps of low values. On the left, what I found like this next year : five 10 eurocents Marianne des Français with a little too much ink. It's very clear on top of the title's letters and on the money denomination.
Look for them when you need to learn.
Forget them and you'll find.