(Pardon if i misuse some jsp/servlet jargon slightly.. I'm not an expert)

When caching a page and using the <cache:addGroup> -tag i noticed that the HashSet that contains the cache-keys depending on this group contained many more entries than they should.

Apparently instances of the cache-tag objects are resused by the web-container (In my case Tomcat). This is "correct" behaviour for so called "classic tags". Whenever an instance is reused, first a call to doStartTag() is done, and here any instance-variables you dont want reused should be cleared.
Since the groups-variable was never cleared, the list of groups that depended on a cache-key just kept growing.

Suggested fix: add the line
  groups = null;
to the beginning of CacheTag.doStartTag()
Perhaps/probably also some other instance variables should be reset here...

Workaround: add the string 'groups=""' to the <cache:cache> tag. This forces
a re-initialization of the groups-variable.