NOTE: This information only applies to Tapestry 4. For current information on Tapestry 5 see http://tapestry.apache.org/
Here's a nice little trick that will pinpoint exactly where Visit or any other ASO is created.
In the constructor of your ASO, add
throw new ApplicationRuntimeException("Creating visit");
In this way, whenever that ASO is created, you'll be presented with Tapestry's detailed exception page (it even displays the line number and the code that triggers the creation of the ASO)