Sans is a pretty skilled mage, but that doesn't really mean that when trying to do three different spells at the same time he's incapable of fumbling. He was hoping his gravity attack would buy himself enough time to send Frisk away and get himself some distance - when it reality, he finds himself too magically preoccupied to calculate a shortcut.

He really only sees the mud when it's already going for him. The braided strands of water and earth wrench him towards the ground and down into the mud, the saturated land giving way under the force of his body and effectively half burying him in the same stroke. The upside is that the mud isn't hard enough to crack his fragile bones in its present state. The downside is that, in a matter of moments, he's been resigned to pretty much drowning in it.

He doesn't have the time for much banter besides a rasping sound of objection through his teeth, before calling on his blaster again with what remains of his focus.

"No offense," he manages, trying to pull his face and hand away from muck he's sinking into long enough to aim properly. Now he's shooting for keeps.