Stephan is told the execution he is meant to stop is scheduled to happen 15 days after that night's masked ball, which date is shown on an invitation. The date on the execution order seen later is exactly consistent with this 15-day deadline. However, the invoice Stephan views in Paris states that the candlesticks were sold to a London auction house one day AFTER the execution date.
When the Baron is in bed reading the letter and getting details from Mr. Korum, the amount of brandy in his glass diminishes significantly inexplicably without him drinking it.
All kept saying Petersburg instead of Saint Petersburg.