What rules in the patimokkha could lead to a monk’s expulsion from a monastery?

In the pātimokkha, the monastic code governing Buddhist monks, there are four specific offenses—often referred to as the gravest sins—that result in immediate and permanent expulsion from the monastic order. These four prohibitions are:

• Sexual intercourse
• Theft
• Murder (or the taking of human life)
• Exaggeration of one’s miraculous powers (falsely claiming spiritual attainments)

These rules constitute the first section of the pātimokkha. While the total number of rules varies slightly between traditions—such as 227 in the Theravāda canon and 250 or 253 in others—these four foundational prohibitions remain consistent across the major schools of Buddhism as the primary grounds for expulsion.12