Fairy Tail x Rave (Chapter)

On April 2011 (a week before Japan's Golden Week), Hiro released a special crossover featuring both leads from Fairy Tail and Rave Master.

Full Summary
The story is about Natsu, Lucy and Happy on an assignment to stop a wild mage who goes nuts in casinoa. Upon arriving in town, Natsu gets lost from his team and winds up meeting Haru. At the same time Lucy and Happy meet Elle, Griff and the Rave Master's Plue. While both parties initially get along at first. Upon hearing the mage description that suspiciously matches Elle, the Fairy Tail mages begin to suspect its her. Natsu and Haru begin to fight (Natsu going off his suspicion, Haru wanting to protect Elle) and nearly wind up tearing up the town. Just as the fight reaches a fever pitch, they're both stopped by the wild mage who turns out to be Erza (who was oblivious of her actions). Both parties go their seperate ways, though Haru and Natsu think the other is an evil person now.

Judging by the dialoge the crossover takea place in the middle of the Rave series as Elle still has amnesia and Haru still has his old sword.