"EB Garamond" font combines ʿ character and the preceding letter.
Consider following document:
\documentclass{article}
\usepackage{fontspec}
\setmainfont{EB Garamond}
\begin{document}
aʿ bʿ cʿ dʿ eʿ fʿ gʿ hʿ iʿ jʿ kʿ lʿ mʿ nʿ oʿ pʿ qʿ rʿ sʿ tʿ uʿ vʿ wʿ xʿ yʿ zʿ
\end{document}

How can I prevent combing ʿ (MODIFIER LETTER LEFT HALF RING) and the preceding letter? (In above images, only b,d,i,j are rendered correct.)
Update After testing more than thirty, this is the only font has this issue. Unfortunately this is only suitable form for a paper that contains a lot of Unicode glyphs. ◕︵◕

a\kern0ptʿwould work/ – David Carlisle Apr 11 '13 at 11:13ʿis U+02BF "MODIFIER LETTER LEFT HALF RING" so that's the expected behavior if the font has the information for modifying the preceding letter. – egreg Apr 11 '13 at 11:56:)– خالد حسني Apr 11 '13 at 19:18