There have been times when Harley Quinn has disappointed her closest companion and future lover Poison Ivy but posing as her might be Harley's greatest betrayal ever. Ivy wasn't angry that her ally had imitated her, but it was the reason why Harley used Poison Ivy's likeness that really hurt her.
If there's one thing comic fans know about Poison Ivy, it's that she is not a fan of humanity. Dr. Pamela Isley has often been portrayed as a misanthrope who regards mankind as a blight on the natural environment of Earth. But for all of Ivy's hatred for others, she has a strong love for Harley Quinn. Somehow, Harley broke through the walls Ivy built up and proved to the flora-themed villain that not all humans are bad. Ivy might still harbor animosity towards most people, but she's always had a special place in her heart for Harley.
But even Ivy has her limits for her beloved Harley and the latter found out what they were in a major dressing down. In Harley Quinn #1 by Karl Kesel and Terry Dodson, the Joker has once again been caught and tossed into Arkham Asylum to rot. Not one to let that happen to her Puddin’, Harley Quinn infiltrates Arkham in an ingenious way. She disguises herself as Poison Ivy and gets herself locked up. When the guards come to check on a presumably powerless Ivy, Harley easily dispatches the guards and makes her play to spring the Joker from Arkham. While Harley is successful in her plan, word of Harley's stunt travels through the grapevine and eventually reaches her inspiration's ears. Ivy confronts Harley over her stunt and Ivy is once again forced to spell it out to Harley that no matter what she thinks, the Joker does not love her.
It’s a frustrating situation for Poison Ivy. At this point in time, Harley was so gaga over Joker that nothing could convince her that the Clown Prince of Crime was bad news. While Harley would eventually wise up and break it off with the Joker for good, Ivy had to have this conversation with Harley dozens of times. But what really incurred Ivy's wrath this time is that Harley made Ivy an unwilling part of Harley's Joker obsession by stealing her friend's identity.
Ivy is indeed an individual few would call a ‘people-person,’ but while her distaste for humanity is one thing, she has a special hate for the Joker. The villain tormented and kept the one person she had fond feelings for trapped in a vicious cycle for years. So, it's understandable that Ivy was especially peeved that Harley would actually imitate her in order to free the Joker from Arkham. For Ivy, it showed how little Harley took Ivy seriously and how little impact Ivy's words had on her friend. While Harley Quinn eventually wised up, her Poison Ivy dress-up remains her biggest friendship faux pas.