An ActionScript Compiler bug that I discovered today (that has been open for 5 years according to Adobe! http://bugs.adobe.com/jira/browse/ASC-2555;jsessionid=0DF58E72CC5A07E2AF4CAC97C7A22005?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel) whereby a compliation error caused in one timeline script will cascade to another without an appropriate warning as to where it was generated.
In other words you may get a compilation error that states you are missing a parenthesis or similar before the first line of code in a layer of your timeline script, which of course makes no sense. However since all layers are compiled into a single pseudo-document class at compile-time (even if you have a Document Class) as far as it’s concerned the preceeding frame containing a script is glued onto it, so the error will have been generated from a timeline script in a layer higher up.