Falling scenery, curtains that won't go up and revolving stages that refuse to turn have caused a series of mishaps in London theaters in the last 10 days, prompting a safety investigation. The latest incident occurred Thursday night at a gala opening at the Royalty Theater of ``A Clockwork Orange.'' The curtain refused to go up for the second half of the Royal Shakespeare Company production. The star-studded audience downed free drinks offered by the embarrassed management as stage staff struggled for 15 minutes before getting the curtain to go up. On Monday, the Stephen Sondheim musical ``Sunday in the Park with George'' at the Lyttelton Theater was halted in mid-performance when a wooden tree, part of the scenery, fell from a hook as it was being carried off the stage. It narrowly missed an actor. Performances at the Lyttelton were canceled the next day. ``It is an extremely complicated set. It has simply been bad luck, but we can take no chances,'' said a theater spokesman. It was the second mishap to hit the musical. On April 23, an iron bar fell, but nobody was injured. On April 24, a hydraulic jack at the Olivier Theater broke down, preventing the revolving stage from turning. The play ``The School for Scandal'' was canceled in mid-performance when stage staff failed to get it working again. The comedy was playing to a packed house. Tickets were refunded. The actors' union Equity said in a statement this week that a working party comprising representatives of the union, stage staff and theater managers had been set up to investigate safety on stage as a result of the incidents.