When two Webengine apps runs on the same Nuxeo server and share some comme resource filenames (for instance skins/base.ftl, it happens that on heavy load the first app uses the base.ftl of the second one which leads to inconsistencies. For instance Root doesn't resolve to the same object, doesn't have the same method.
This problem *seems* also to happen to new Webengine apps. Symptom is that base.ftl of default apps may be use the first time an application is accessed (it needs a refresh of the UI to get the real one).
A workaround is to rename on the files to that no collission happen. (It seems to happen only on the extends directive)