DADA Position - Quirrell and 1-year Curse

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No: HPFGUIDX 133321

I just pulled out SS to start my re-read of the series after my first read through HBP, and 
pounced on the meeting with Quirrell. I had been under the impression that Quirrell had held 
the position for some time, and I ran across the passage that had created this impression. 

"He was fine while he was studying out of books but then he took a year off ter get some 
first-hand experience.... They say he met vampires in the Black Forest, and there was a nasty 
bit o' trouble with a hag -- never been the same since. Scared of the students, scared of his 
own subject..." (pp. 70-71 US paperback)

How could he /now/ be scared of his subject whereas he wasn't before, if he hasn't taught 
DADA before? If he's taught DADA before, then Dumbledore can't be right in saying that he 
hasn't been able to keep someone in this position for more than a year. Any thoughts?

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