What very Bad Thing Does Harry Do in HBP?

[Vivamus]

No: HPFGUIDX 123480

Please excuse me if this is incorrect, but I think someone in here posted a
recent quote from JKR, to the effect that Harry does something very bad in
book six. 

Assuming that is a quote from JKR (and therefore canon), it has occurred to
me that IF SS is indeed a double agent for DD, pretending to be a DE, Harry
could discover part of SS's activities, wrongly assume that means he is
still a DE, and either betray SS directly, blowing his cover, or else
completely mess up whatever SS is trying to do for DD at the time.  It would
almost certainly cause someone's death to be as serious as the quote
suggests, but I don't think that someone would be SS -- there is too much
tension in their relationship to resolve before book 7.  (Gasp -- what if it
indirectly causes DD's death?  What a beautiful plot situation that would
create.)

I still don't like SS as a double agent, for reasons over-dwelt upon here,
but if he is, doesn't that make an interesting theory?

Vivamus