Numerous bugs have reported a similar problem 4137112, 4171490, 4158240 and 4176491. All of these bugs have been closed for one reason or another, either as duplicates or fixed in certain versions of the plugin. However, this bug still does not appear to be fixed. The customer (and my tests) used plugin 1.2.1 on windows NT 4.0.
  In the attachments is a test case. The test case has a directory SunMemoryProblem/MemoryProblem which contains the file readme.html. In it are directions for reproducing the problem. Basically, load the file StartTest.html and then click on the button. Observe the memory amounts printed into the plugin console. Click back and the click the button again causing the applet to be reloaded. Do this repeatedly and observe the memory increase each time until finally you run out of memory.
  This seems to happen for any applet. And the 4 bugs mentioned above described very similar problems. However, the problem still occurs.