Create a Resource with a Method that has a query parameter that is an Enum.
Call that method and for the enum, pass an invalid value (one that isn't an existing value for this enum).

A NotFoundException will be thrown with the following error message:

RESTEASY003870: Unable to extract parameter from http request: javax.ws.rs.QueryParam("your_enum_param") value is {1} for <your_method>

Where "your_enum_param" is your Query Parameter and <your_method> is the Resource's Method.

The main issue here is that

does not contain the invalid value. This seems like a String format error.

Another issue is that this results in a NotFoundException. I would have expected a more specific exception that would have allowed me to return a more interesting error message. I'd love to have the value passed and the Class<> of the enum that was requested. This would allow me to return a message saying "Value X for parameter Y is not valid. Valid values are: A,B,C".