- Affected App
- WoltLab Suite Forum
You can reproduce this easily here on Woltlab.com 
- Step 1: Be a guest
- Step 2: Click on a quote button
- Step 3: ???
- Step 4: Profit.
(All jokes aside 
, the end result is shown in the screenshot)
Display Spoiler
Ideally, one would expect it to either quote (if you allow guest to quote). OR ask your to log-in and/or register.