Flickering Circles

In manga and anime, circles or sweat droplets drawn flickering erratically around a character's head mean that they're panicking, flustered, or agitated.

They're specially used when the character is worrying about something, or trying to explain something hard to explain to someone, or when they're caught red-handed doing something and someone is currently angrily walking toward them.

Hiraga Saito 平賀才人, example of sweating circles.

Character: Hiraga Saito 平賀才人
Anime: The Familiar of Zero, Zero no Tsukaima ゼロの使い魔 (Episode 3)

Examples

For reference, some examples:

Watanuki Banri 渡狸卍里, example of sweating circles.

Left: Watanuki Banri 渡狸卍里
Right: Shirakiin Ririchiyo 白鬼院凜々蝶
Anime: Inu x Boku SS, 妖狐×僕ＳＳ (Episode 4)

Hakubi Ryouko 白眉魎呼, Masaki Tenchi 柾木天地, example of character sweating in panic.

Left: Hakubi Ryouko 白眉魎呼
Right: Masaki Tenchi 柾木天地
Anime: Tenchi Muyou! Ryououki 天地無用！魎皇鬼 (OVA, Episode 1)

Context: small sweat drops drawn on hair are another sweat-related symbol.

Yoshida Yuuko 吉田優子, Shamiko シャミ子; Shamisen シャミ先; and Hinatsuki Mikan 陽夏木ミカン, example of "X eyes," batsu-me バツ目.

Left: Yoshida Yuuko 吉田優子, Shamiko シャミ子
Middle: Shamisen シャミ先
Right: Hinatsuki Mikan 陽夏木ミカン
Anime: Machikado Mazoku まちカドまぞく (Episode 10)

Left: X_X.
Right: >_<.

Shirogane Miyuki 白銀御行, Fujiwara Chika 藤原千花, Shinomiya Kaguya 四宮かぐや, example of "bunny ears," usamimi うさみみ.

Left: Shirogane Miyuki 白銀御行
Middle: Fujiwara Chika 藤原千花
Right: Shinomiya Kaguya 四宮かぐや
Anime: Kaguya-sama wa Kokurasetai? ~Tensai-Tachi no Ren'ai Zunousen~ かぐや様は告らせたい？～天才たちの恋愛頭脳戦～ (Season 2) (Episode 3)