Berlin's Spielwiese convention last July had over 200 prototypes on display. I mean, idk if it's just me, but calling it an accidental detour might be underselling how common those testing sessions are. Most designers there are actively hunting for playtesters, so getting roped in is almost expected. Maybe from my visits, the real highlight is often the after-parties where designers brainstorm over beers. Still, prototype testing can be fun if you're into rule mechanics and balance discussions.
Thought prototype testing sounded boring as hell. Getting roped into one last year actually made me appreciate the chaos, though. Now I kind of look for those sessions.