1991 Johor State Legislative Assembly opening incident (reported by Wikipedia, single source)
Wikipedia reports that on 20 June 1991 at the opening ceremony of the Johor State Legislative Assembly, the Sultan of Johor punched and slapped Pang Hok Liong and fellow DAP assemblyman Wong Peng Seng, and the then-Crown Prince Ibrahim Iskandar kicked Pang in the legs; the DAP central committee confirmed the incident.N8
Status: No police report was made and the supplied records show no prosecution, court case, charge or finding; the account rests on a single Wikipedia entry.