Let users foobar, barfoo and fooo be subscribed to email notifications on changes of page X.
Suppose user foo changes X, then fooo gets notified but not foobar and barfoo since foo is a substring of foobar and barfoo.
The problem lies within subscription_addresslist and subscription_regex in subscription.php, where the following regular expression is built to match subscribers in the .mlist files:
/^((?:(?!foo)\S?)+)(?: every(?: (\S+))?)?$/
A possible correction would be to match
/^((?:(?!foo ))\S+)(?: every(?: (\S+))?)?$/