Loanwords
Consonant clusters occurring in loanwords do not necessarily follow the cluster limits set by the borrowing language's phonotactics. The Ubykh language's root psta, a loan from Adyghe, violates Ubykh's rule of no more than two initial consonants; also, the English words sphere /ˈsfɪər/ and sphinx /ˈsfɪŋks/, Greek loans, violate the restraint (or constraint, see also optimality theory) that two fricatives may not appear adjacently word-initially.
Read more about this topic: Consonant Cluster