When you enter an invalid input into a block. For example leaving the field of the wait seconds block empty. You get a prompt that tells you that you have made an invalid input. Though once you fixed the invalid input, the text will go away immediately but the black box behind the text takes a bit longer to go away (about 7 seconds). They should both go away at the same time once you have fixed the error in your code.
A video of the bug : https://imgur.com/a/ksTh3TC