It happened fairly often in the manga. Usually was Sayaka the one did the unwilling distraction — either because her clothes were torn or because she was involuntarily naked —, but sometimes it was done by enemies to distract Kouji. Like the Gamia sisters...
One of those scenes was animated in Mazinkaiser. In the Beach Episode, Kouji accidentally undoes Sayaka's top*
he was trying to grab her shoulder and missed
; she's actually more upset by Boss getting a look at her breasts than she is by Kouji, even though he weaponizes her boobs in order to escape the enemy fortress.
Great Mazinger: Just how many times Boss ran into an obstacle because he saw Jun walking by and he stopped looking forward while he walked?
Happens a lot to Maeda in Ai Kora, who has a tendency to go nuts for any woman who has at least one of what he considers "ideal parts". So far the only girls in his "harem" to consciously manipulate this fact are Ayame and Yukari.
Sayonara, Zetsubou-Sensei has a segment on this (or rather, getting distracted in general, and forgetting the point of what you're watching) that involves an invading alien leader getting completely sidetracked from his invasion by gratuitous fanservice... which the viewer is also shown, and which generally does succeed in distracting the viewer from the actual plot until the next obvious reminder. Other non-sexy distractions include showing the credits onscreen at moderately high speed without stopping the action or dialogue in any way. It works, too.
Fairy Tail: This happens to Juvia in a side chapter where she's supposed to be using her magic as a water slide attraction for Lucy, who ends up tumbling ungracefully into the water when Juvia sees Gray's "glorious body".
In Change 123, this trope is played (at least) twice:
Ozuma, during the judo match with Mikiri, suddenly sees that under her judo-gi Mikiri has nothing but some skimpy underwear. She quickly manages to score a small point because of his distraction.
On the sports festival at school, during the "battle on horses", Hibiki and Ginga jump towards Izuru to take away her headband. Izuru manages to evade Hibiki's hand, and so instead of the headband, Hibiki tears away Izuru's clothes. This results in Hibiki's and Ginga's "horses" to be distracted by Izuru's breasts, and so unable to catch their "riders".
In Star Driver, this happens to Okamoto during her fight with Takuto. Much to the disbelief of the observers. And her opponent.
Mayumi-chan from Kuuchuu Buranko is there for that very reason.
In one episode of Panty & Stocking with Garterbelt, Panty and Stocking go job hunting. One part of the job montage features them directing traffic, but their tight cop outfits cause more accidents than the girls prevent.
In After School Sex Slave Club one of the cheerleaders/sex slaves is told not to wear any underwear during a baseball match. The opposing team lost by paying too much attention to her.
In a similar but less naughty variation of the above, Urd turns the tide of a softball match by showing up in a uniform two sizes too small without a bra, destroying the focus of the all-male team she and the rest of the cast were playing against.
This is why Plica doesn't remember much of her piano lessons.
In Busou Renkin, when Tokiko is forced to wear a two-piece bathing suit, she spends the whole long beach scene chewing out other people whom she accuses of staring at her bellybutton. She even yells at the readers.
In the Love Hina manga, when Kitsune, Motoko, and Mutsumi are trying to get a ride to the airport, Kitsune lifts up Motoko's shirt to entice travelers... and promptly causes a 20-car pileup.
In Tora Dora, this was Played for Drama as all the boys were too distracted looking at Ami in her bikini to notice that Ryuugi was accidentally knocked out and fell into the pool, leaving Taiga (who cannot swim) to go after him. She promptly calls the boys out for their behavior.
Durarara has a rather unusual recurring example: Anri herself doesn't have any notable attraction to Shizuo Heiwajima, but the Evil Weapon that she serves as a host to is completely Yandere for him. As a result, Anri's mind is flooded with obsessive thoughts and... uncomfortable physical sensations when she so much as looks at him, and she has to concentrate to push them down.
Particularly hilarious example in Vipter GTS when we have a nerd summon a succubusto grant him a wish. When he sees her, he quickly can't think of much beyond her sexiness and wished for her to have sex with him. The succubus doesn't and likes the idea.
Happened in Yes! Pretty Cure 5 concerning the school reporter. Whenever it's possible the girls were caught on camera, Mr. Natts would show up and she'd focus more on him than the girls in terms of article importance.
The minor character Ran Fan tried to distract the equally minor character Nam during their fight in the First World Martial Arts Championship by stripping down to her underwear. It worked perfectly until Nam realized he could just close his eyes. The entire sequence was cut from the Toonami broadcast version.
During his fight with Jackie Chun in the same tournament, Krillin decides the only way to beat a Dirty Old Man is to...throw a pair of Bulma's panties at him and kick the old pervert while he's distracted.
He's not the first great warrior (or the last) to be brought low by a girl's panties—
