Stop in /usr/ports/databases/mysql50-server.
xt436# cd /usr/ports/databases/mysql50-server/
xt436# make install WITH_XCHARSET=all clean
===> mysql-server-5.0.95 cannot install: MySQL versions mismatch: mysql55-client is installed and wanted version is mysql50-client.
*** Error code 1
Stop in /usr/ports/databases/mysql50-server.