when did the prank take place?

[snapesmate]

No: HPFGUIDX 83365

--- In HPforGrownups at yahoogroups.com, "kiricat2001" <Zarleycat at a...> 
wrote:
<snip>  Could Sirius come up with a plausible (to Snape) story 
> that he could tell with a straight face that Snape wouldn't treat 
> with the utmost suspicion?  Doesn't seem likely.  
<snip>
> Marianne

Perhaps Sirius and James had a "stage whisper" type conversation that 
they let Snape hear "accidently on purpose" so he would go and 
investigate.  I cannot remember the exact quote from the book, but 
Sirius mentioned something about setting Snape up because they knew 
he was "too nosey for his own good" (paraphrasing).  Then, of course, 
James decided it was wrong and stopped Snape.  
Lynnette, who thinks Snape is a good guy, even if he is not a nice 
person!