The mysql2 gem in version 0.3.13 might break while compiling on older patch releases of Ruby 1.9.3 within rvm:

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*** [err :: server] ruby: symbol lookup error: /path/to/deployment/shared/bundle/ruby/1.9.1/gems/mysql2-0.3.13/lib/mysql2/mysql2.so: undefined symbol: rb_wait_for_single_fd
*** [err :: server] ruby: symbol lookup error: /path/to/deployment/shared/bundle/ruby/1.9.1/gems/mysql2-0.3.13/lib/mysql2/mysql2.so: undefined symbol: rb_wait_for_single_fd

Fixating mysql2 to version 0.3.11 helped.