. . . and swears, because now his shoes are a mess.

Should you use “into” or “in to?” Well, it depends. (It always depends, doesn’t it?)

You walk into a building, or into a room. You’re moving; you are changing your location from outside to inside.

However, if there’s a pizza in the room you walk into, you “walk in to a pizza.” You walk in to the presence of pizza (presumably on a table or counter, not the floor).

I will quote Garner, so you will know I’m not blowing smoke: “These prepositions aren’t ordinarily interchangeable, and care must be taken in choosing between them: in denotes position or location, and into denotes movement. Thus, a person who swims in the ocean is already there, while a person who swims into the ocean is moving from, say, the mouth of a river. There are many exceptions, however, especially with popular idioms <go jump in a lake>.” (Garner’s Modern American Usage, page 450)

Similarly, you might say “I ran in the mall last week,” meaning you went for a run inside the mall (one would hope the mall sets aside times for such activity, so you weren’t running over little old ladies with shopping bags). “I ran into the mall last week” means something entirely different. Did you “run into the mall” because someone was chasing you from the parking lot? Or did you “run into the mall” to pick up a last-minute gift for someone? I hope these examples help delineate which preposition to use. People “walking into donuts” are likely to have pretty crumby shoes (as opposed to “crummy” shoes, which aren’t the same thing at all).