I am using LibreOffice 3.3.3 on Kubuntu Natty and 3.4.2 on Windows XP. On both installations I observe the following bug:

What I did:

I typed the Tamil word சித்திரை (name of a month) into an LO application. I used the left- and right-cursor keys to navigate the word.

Expected behaviour:

The "correct" native-user-perceived grapheme cluster split-out is: சி|த்|தி|ரை (ci|t|ti|rai) as in Tamil, vowelless consonants are considered independent grapheme clusters on their own. So it should be possible to place the cursor at any of the above positions indicated by the |, especially between த் and தி.

Actual behaviour:

LO analyses the word as சி|த்தி|ரை (ci|t.ti|rai) and does not allow cursor placement between the த் and தி. When my cursor is to the left of த் and I press the right-cursor key, the cursor moves to the right of தி, and vice versa with left-cursor key.

Background:

Generally in Indic scripts like Devanagari, a vowelless consonant would be taken into the same grapheme cluster as a following consonant, as mostly ligatures or conjoining forms between the consonants will occur in such cases.

For example, the same word presented in Devanagari would be analysed as चि|त्ति|रै (ci|t.ti|rai) since the vowelless "t" ligates (or in the absence of ligature in a font, takes a conjoining form) with the following consonant. 

However in Tamil, vowelless consonants never ligate or form conjoining forms with following consonants. (The only exception is க் ligating with ஷ to form க்ஷ.) 

Therefore vowelless consonants in Tamil are always perceived by native users as grapheme clusters on their own. Therefore a native user expects to be able to place a cursor immediately before or after a vowelless consonant, so: சி|த்|தி|ரை 
 
Other applications (like for example Firefox which I am using now to report this bug) correctly treat த் and தி as separate grapheme clusters in the word சித்திரை.

Note:

This faulty behaviour is seen for all CONSONANT + VIRAMA + CONSONANT sequences in Tamil in LibreOffice. The one obvious exception is க்ஷ i.e. KA + VIRAMA + SSA where the ligature is formed and so the native user does *not* expect to place the cursor in the middle of it.