It appears there have been instances of connection pool leaks in C3P0 0.9.0.4, according to this issue in Hibernate:

http://opensource.atlassian.com/projects/hibernate/browse/HHH-2032

We should upgrade to 0.9.1 or later to fix this. (Apparently the c3p0 case number is 00010976, but I can't find that anywhere.)

The message in the logs looks like this:

14:43:58,253 WARN BasicResourcePool:1222 - com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@edf4c8 – Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (30).

The upgrade should be backwards-compatible, so the c3p0 JAR can be dropped into Confluence 2.7.x instances as well to fix this problem.