In Gotham City Sirens #23, Catwoman said to Batman (Dick Grayson) "I know what you did before you won the cowl" 
and then another time, as a thought: "...Do you know what Dick did?"  
 
WHAT DID HE DO?!?!!? I'm extremely confused!!  
Please fellow comicbook readers! Tell me! what did Dick Grayson do that has Catwoman's panties tied in a bunch?!!

Maybe just him being Robin?

...... It's a secret you are NOT ALLOWED TO KNOW.

She knows how he broke Barbara's heart by sleeping with starfire O_O 

Well Dick has done a number of things before he won the cowl.  If I had to take a guess... and this is a shot in the dark because of the Azreal thing going on with the judgement of Gotham, is Dick beat the Joker to death, so techinically he's taken  a life.  He did it because he thought that the Joker had fed Tim Drake to Killer Croc and his anger got a hold of him.  Bruce showed up to revive the Joker, but that doesn't change the fact about what was done.  The writers could very well be remembering that and bringing it back up. 
 
On a side note, screw Barbara.  She's put him through more emotional turmoil than any other woman's he's been with.

There is already a thread about this

revealed in detail in GCS#26 (god I hate that this series came to an end)

Beat Jason Todd?

I may be wrong but i believe what he did was have selena keep tabs on ivy and harley, and possibly was responsible for their release.

@Primmaster64 said:

Beat Jason Todd?