Nick Reiner Interrupted Bill Hader at Conan O'Brien's Xmas Party, Report Alleges

Nick Reiner was acting out of place at Conan O'Brien's Christmas party hours before his parents, Rob and Michele Reiner, were found stabbed to death in the couple's home ... and now there's a report of a strange interaction that concerned partygoers.

According to NBC News, Nick allegedly interrupted comedian Bill Hader mid-conversation during the holiday gathering Saturday night in L.A. The outlet reports Hader told Nick the conversation was private ... and Nick then slipped into a thousand-yard stare, before "storming off."

As we reported ... Nick also got into a loud shouting match with his father at the same event. In recent months, Rob and Michele were struggling with how to deal with Nick as he continued to battle mental health issues and alleged substance abuse. Rob reportedly brought Nick to the party to keep an eye on him.

Hours after the party, on Sunday afternoon, Rob and Michele's daughter Romy discovered their stabbed and bloodied bodies at their Brentwood house. Rigor mortis had already set in, meaning they had been dead for hours.

Romy is the one who called the police and told them her brother Nick -- who'd been living at the house -- was "dangerous" and should be considered a suspect.

About 6 hours later, Nick was located and arrested on suspicion of murder ... he's currently being held on suicide watch at Men's Central Jail in L.A. without bail.

We've reached out to Bill Hader ... so far, no word back.