Mosquito girl was one of the first monsters we saw Saitama apparently killed. Later in the manga, she reappears. How did she survive that Saitama punch which apparently completely smashed her, and converted her in a large splash of blood over a building?
When Saitama slaps her it seems he actually sent her blasting off Team Rocket style rather than killing her outright. This is the relevant panel in the manga (Ch6),
I've circled the person shaped speck (which is easy to overlook among the rubble of the building Saitama bisected using Mosquito Girl as a projectile).
The anime version is similar (S1 Ep2),
but instead of launching Mosquito Girl through the building he launches her past it (I've also circled the person shaped speck here) and instead there's a massive blood splatter as though he splattered her against the building. The blood splatter is anime-only and I'm pretty sure its her blood sac getting splattered rather than her.
Note the giant red blood sack in her anime design and how it got bigger after her mosquitos came back to her with a bunch of blood. It would explain how a person shaped speck can get launched past the building while also creating that wicked blood splatter.
The webcomic is actually a lot more clear about what happened (Ch6),
With the simpler art (i.e. lack of rubble / blood) its clear Saitama launches her through the building, whether or not that's meant to be survivable is anyone's guess but its (probably) better than getting obliterated.