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I was catching a Caterpie, but my game crashed after 'Gotcha!' appeared (little pokéball spinning at the top left), so I killed the app and launched it again.
Now despite the 'Gotcha!' message, I cannot see the Caterpie in the list of Pokémon I've caught recently (the last one was a Weepinbell). I'm level 12 and this is the second time that this has happened.
I was using WiFi with my Sony C4 (Android). As far as I know, my WiFi is working fine and I don't have any GPS problems.