At the left side of the screen, upper platform. A goo dropped next to this edge becomes permanently stuck. The cursor shows the 4 black activation marks (i.e. the goo is there) but no amount of coaxing (multiple clicking, click and move etc) will shift the blighter. Can be worked round by blocking access to the pit, but seems odd when the right side equivalent pit spits out dropped goos. I'm on version 1.3, PC version. I was prising off the last of the black goos from the initial structure and found the edge of the screen was one solution only to lose out to the pit.