Yep! Under the right conditions, the output side of the coupler will have as much as 10 kV of RF, which will arc to just about any material, even plastic.
Mounting it in the middle also makes a proper ground connection next to impossible. The ground connection (not ground plane mind you) needs to be short; inches not feet.
Go to my web site, and read the Antenna Couplers article.
Alan, KØBG
www.k0bg.com