Confluence locked up for a customer, and the thread dump showed all the threads either waiting for a database connection, or sitting like this:

"http-80-Processor14" daemon prio=5 tid=0x068f78b0 nid=0x904 in Object.wait() [7bfe000..7bffdb8]
at java.lang.Object.wait(Native Method)

• waiting on <0x1fad0d60> (a java.awt.MediaTracker)
  at java.awt.MediaTracker.waitForID(MediaTracker.java:631)
• locked <0x1fad0d60> (a java.awt.MediaTracker)
  at java.awt.MediaTracker.waitForID(MediaTracker.java:586)
  at com.atlassian.core.util.thumbnail.Thumber.getImage(Thumber.java:237)
  at com.atlassian.core.util.thumbnail.Thumber.createThumbnail(Thumber.java:168)
  at com.atlassian.core.util.thumbnail.Thumber.retrieveOrCreateThumbNail(Thumber.java:131)
  at com.atlassian.confluence.pages.thumbnail.AbstractThumbnailManager.getThumbnail(AbstractThumbnailManager.java:79)
  at com.atlassian.confluence.pages.thumbnail.DelegatorThumbnailManager.getThumbnail(DelegatorThumbnailManager.java:54)
  at com.atlassian.confluence.servlet.download.ThumbnailDownload.getAttachmentStream(ThumbnailDownload.java:25)
  at com.atlassian.confluence.servlet.download.AttachmentDownload.getStreamForDownload(AttachmentDownload.java:83)
  at com.atlassian.confluence.servlet.download.ServeAfterTransactionDownload$StreamResultCallback.doInTransaction(ServeAfterTransactionDownload.java:128)
  at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:114)
  at com.atlassian.confluence.servlet.download.ServeAfterTransactionDownload.getStreamInTransaction(ServeAfterTransactionDownload.java:40)
  at com.atlassian.confluence.servlet.download.ServeAfterTransactionDownload.serveFile(ServeAfterTransactionDownload.java:46)
  at com.atlassian.plugin.servlet.BaseFileServerServlet.doGet(BaseFileServerServlet.java:77)
  at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
  at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.confluence.util.LoggingContextFilter.doFilter(LoggingContextFilter.java:46)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:182)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:159)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.confluence.util.ClusterHeaderFilter.doFilter(ClusterHeaderFilter.java:35)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:122)
  at com.atlassian.core.filters.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:27)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.confluence.util.RequestCacheThreadLocalFilter.doFilter(RequestCacheThreadLocalFilter.java:25)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.core.filters.gzip.GzipFilter.doFilter(GzipFilter.java:53)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
  at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
  at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
  at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
  at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
  at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
  at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
  at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
  at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:667)
  at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
  at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
  at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
  at java.lang.Thread.run(Thread.java:534)

The full thread dump is attached.

I couldn't reproduce this with his images, but its happens frequently for him.