I discovered this on 9.0.5, but the issue seems to be caused by Java 8, so it doesn't matter which version of TD is used.
I tried to import data from REST ws and when I added some parameter (name of parameter is "new") I got following error:

Illegal character(s) in message header field: rest_param:new

This seems to be problem only with Java 8. It works with Java 7.

Marked this as blocker, because user cannot import from REST if he wants to use some parameters during import. After some googling I found out that this could be an issue in Java not in Designer.

Java version: 1.8.0_65, vendor: Oracle Corporation