There are innumerable cases of overnight risks that have ruined the option seller. The most famous case is that of Barings in 1995. Their star trader, Nick Leeson, had sold calls and puts on the Japanese Nikkei index expecting the index to be in a range. However, when the Kobe earthquake struck Japan, the Nikkei crashed resulting in huge losses on the option sell trade. The trade resulted in a huge $1 billion loss which wiped out the entire capital of Barings bringing the 200 year old bank to bankruptcy. Even in India events like upper circuits and lower circuits have created huge overnight risk. It is advisable to avoid heavy short options position over long weekends, especially when scope for volatility is evident.