The latest released version of scriptaculos (1.8.3) has a really old memory leak. In short, Autocompleter creates a lot of event handlers and never removes them. Given that Checkvist will use tag autocompletion rather intensively, I've decided to fix this problem.
My solution is attached to the issue at the lighthouse and also available in my fork of scriptaculos.
May be this fix will be helpful to someone else :)