I believe the poster was referring to the practice of testing if an app was installed by calling [UIApplication canOpenURL:] —- as I recall Twitter was found to check for hundreds of different apps, to feed into their ads and analytics business, and Apple later changed it to only be callable 50 times.

Nit: for 50 different schemes, not 50 times.

And you have to provide those schemes in your App's plist as well if I recall correctly. Gated pretty hard.