Rhotacism is the misuse or over-pronunciation of the letter "r".

Orthoepy

Rhotacism is a medical condition consisting of an inability or difficulty in pronouncing the letter "r".

One famous sufferer is the British TV personality Jonathan Ross, who pronounces his own surname more like "Woss".

Phonetics

In Indo-European languages, rhotacism can be seen in a shift from the sound of "s" to an "r" sound.

Examples of rhotacism can be found in Latin, Germanic (especially the Western branch), Sanskrit and other languages. For example:

English

• was vs were (from Germanic *was vs *wēzun)
• lose vs forlorn (from Germanic *liusana vs *luzenaz)

Dutch

• vriezen vs gevroren (from Germanic *friusana vs *fruzenaz)

Compare also Gothic dags with Old Norse dagr (from Germanic *dagaz)

Latin

• flos (nominative) vs florem (accusative) (Old Latin flosem)
• genus (nominative) vs generis (genitive) (from *geneses, cf Sanskrit janasas)
• corroborare vs robustus (verb from *conrobosare)
• de iure vs iustus (from de iouse)
• ero vs est (from eso)

Sanskrit

Words ending in -s other than -as become -r in sandhi with a voiced consonant:

• naus (before p/t/k) vs naur bharati
• agnis (before p/t/k) vs agnir mata

Slovene

Slovenian rhotacism consists of shift from [Z] (like in English vision) to vibrating [r]:

• moreš from možešь
• kdor from kъtože

Slovenian rhotacism is already visible in the Freising manuscripts, a written document from the 10th century.

The same shift occurred in single words in other South Slavic languages.

Neapolitan

In Neapolitan rhotacism is seen in a shift from the sound of "d" to an "r" sound:

(Italian vs Neapolitan)

• medesimo vs meresemo
• diaspora vs riaspro

and, to a lesser extent, from the sound of an "l" to an "r" sound:

• albero vs arvero
• ultimo vs urdemo

Quenya

Although it is not an Indo-European language, actually not a natural language at all, J.R.R. Tolkien's Quenya swifts -s and -r for historical reasons:

• kár "head" vs kas-, e.g. *casi "heads". Source: The Etymologies (1930s Quenya).
• mar "dwelling of men, the Earth, -land" vs mas-, e.g. *masen "of dwelling". Source: The Book of Lost Tales I (1910s Qenya).