consider the following: tiles is initialized using the org.apache.tiles.definition.UrlDefinitionsFactory default definitions factory.
now, when a request receives and the tiles container is instructed to render a specific tiles definition, the following happens (the interessing parts):
1. invoke 'Definition getDefinition(String name, TilesRequestContext tilesContext)' of UrlDefinitionFactory.
2. from there invoke 'Definitions getDefinitions()' of UrlDefinitionFactory.
3. if the instance attribute 'definitions' is null, from there invoke 'Definitions readDefinitions()' of UrlDefinitionFactory.
within the third call the configuration is actually parsed and the beans/instances/what ever defined within the configuration file(s) are getting created.
unfortunatelly the UrlDefinitionFactory does not synchronize the call to the 'Definitions readDefinitions()' method. the result is, if there are multiple request reaching the servlet container at once and there has no other request been received so long, we get into classical multithreading problematics where each thread is bothering the others and the process of creating the definitions list crashs.