Danielle Staub just got tricked and then treated -- to a lawsuit from a process server who gave the former "Real Housewives of New Jersey" star a serious shock at a Halloween party.
A TMZ photog saw the whole thing go down just minutes ago -- as Danielle was served for a defamation lawsuit her ex-boyfriend Steve Zalewski is bringing against her.
Danielle was just starting down the red carpet at a Halloween event in Woodridge, NJ when the process server walked up with an envelope and said, "Here, Danielle ... you've been served."
Danielle was stunned, but didn't miss a beat -- she simply handed the envelope to her publicist and continued down the red carpet into the party.
According to sources, Zalewski is suing Danielle for defamation
she has repeatedly accused him of secretly shooting a
sex tape of her --
which he denies doing.