snape:second chance?

[revealme4u]

No: HPFGUIDX 115556


I am a new member so don't know about previous posts. And as I was 
reading GoF for the n'th time I came across an incident were Harry 
Hermoine and Ron are discussing Snape.  Harry remarks "if Snape is on 
his second chance, I want to know what he did with his first 
one?"(something like this).
Has it already been discussed about Snape's 1st chance, if so can 
anyone please tell me where to look for?
Else can someone suggest what this first chance was???

Awaiting replies eagerly

vivek