The "Hashtable proxies" instance variable in class java.awt.FilteredImageSource can be inadvertantly corrupted due to certain methods begin unsynchronized. For example, if removeConsumer() is called while startProduction() is executing, the proxies instance var can be set to null. The workaround, briefly, is:

subclass FilteredImageSource and implement the startProduction method to be synchronized.

See the workaround section for a detailed description of the workaround.