Following bug still exists with Liferay 6.1.1. CE GA2 and Tomcat-7.0.27.
It was closed due inactivity.
Liferay stores public render parameters to a map in session.
Therefore, if many requests come to the same page at the same time with different values for the public render parameters, results get mixed up, since all the requests get the parameter values from the last request, since the parameters are stored to session.
Bug might be in the following classes: