typeof x type guard can be unsound in one of the following two cases:

1. typeof x never corresponds to the specified value (e.g., typeof x === 'number' when x has type 'string | boolean')
2. typeof x always corresponds to the specified value (e.g., typeof x === 'string' when x has type 'string')

x instanceof A type guard can be unsound in one of the following two cases:

1. Type of x is not related to A
2. Type of x is A or a subtype of A