It’s not unfitting that Helsinki is a frequently misspelled name, since the city actually has its own slang which does not conform to normal Finnish spelling and grammar rules.
This slangi is a mishmash of colloquial Finnish and words borrowed from other languages: English, Swedish, German and Russian.
Slangi is characterized by a very quick speech, shortened or diminutive forms of words, voiced consonants not commonly used in Finnish, such as d, and a breaking of the rules of vowel harmony.
Often, an extra S is added to the beginning of words in slangi—this would not, however, justify that extra S near the end of our typo Helsinkski!
Pictured above is the Helsinki Cathedral, a major landmark in the city.
(Photograph from http://taivasalla.net)