I think there was discussion about how client library shared library versions during the Lisbon meeting.

From http://dev.mysql.com/doc/refman/5.5/en/news-5-5-10.html

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Incompatible Change: The shared library version of the client library was increased to 18 to reflect ABI changes, and avoid compatibility problems with the client library in MySQL 5.1. Note that this is an incompatible change between 5.5.10 and earlier 5.5 versions, so client programs that use the 5.5 client library should be recompiled against the 5.5.10 client library.

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