Same gravatar is displayed for different users in stackexchange.com filters page under the "Top Network Askers" in the right side bar.

The free hand circled image's source are from https://www.gravatar.com/avatar/?s=18&d=identicon&r=PG&s=30. Seems the gravatar id is missing in the image source.

For an example from the list, the user Mad Scientist's gravatar id should be https://www.gravatar.com/avatar/66c66c4d97a7bc3859a85253f003e1fb?s=18&d=identicon&r=PG&s=30, but is mapped with https://www.gravatar.com/avatar/?s=18&d=identicon&r=PG&s=30, which caused the display issue.