Actor Daniel Hoevels collapsed at Vienna's Burgtheater in Austria with blood pouring from his neck, and the audience even started applauding the spectacular special effects, London's Telegraph said.
It was only when the 30-year-old failed to get up to take a bow that they realised something was wrong, it said.
Hoevels's character was to commit suicide in the drama, a scene that was to be acted out with a blunt stage weapon.
But he'd been given a real blade instead, the Telegraph said.
Police were investigating the incident as a possible murder attempt, and would not rule out the possibility a jealous rival may have switched the blades.
Police were told the knife was bought at a local store, and were investigating whether props staff forgot to blunt the blade for the performance of Friedrich Schiller's play Mary Stuart, about Mary Queen of Scots, the Telegraph said.
"The knife even still had the price tag on it," a police investigator said.
The actor had emergency treatment at a local hospital.
"If Hoevels had hit an artery or cut only slightly deeper, he would have died on stage," a doctor said.
Remarkably, Hoevels returned to the stage the next night with a bandage around his neck.