I hacked the Dutch government T-shirt
The Dutch National Cyber Security Centre (NCSC) sends this shirt to researchers once a vulnerability they reported in central-government systems has been fixed. Black tee, two lines of white text across the chest: "I hacked the Dutch government and all I got was this lousy t-shirt". On the sleeve is a white vertical bar with the crest and "National Cyber Security Centre / Ministry of Justice and Security". You can't buy it. It ships with a letter from NCSC-NL. Variants: a white version with dark text, a hoodie for repeat reporters, and since 2022 a yearly "I am on the [year] Wall of Fame of NCSC" hoodie for the jury's picks.