As of 0.99g, the instruction to get #151 doesn't seem to work. I did these:
-Went to the room of Smile-san, made him/her/it laugh with the telephone effect,
-Interacted with the flashy blob creature in this area,
-Went to the Clock world,
-Went to the Urban Street Area, used the fairy effect to make the NPC fall, then used the turntables, then interacted with the eye creature,
-Found the secret room with the plants, used the glasses effect to see the seed in the middle of the room,
-Went to Urotsuki's Dream Scene, encountered the girl at the bus stop while she was in her detailed form, and interacted with her,
-Went to the Tanuki Room, went to the the empty white room with a Helmet Girl in it (and interacted with her), went to the balcony, interacted the other Helmet Girl here (and got teleported to the Square-Square world),
-Went to the TV area, used the telephone effect to make the screen go nosy, waited out and watched the full screen event,
-Went to the monochrome room, interacted with the eye and watched the fullscreen event.
What else do I need to? The red creature on the wallpaper that peeks out at the top right corner around the mountain looks suspicious, I'm not sure if I've seen it (and apparently I can't find it, even if I spent the last 4 hours solely on exploring the Revenge Building).