I’ve concluded he intentionally messed up the line to prove his point. The verse didn’t just pronounce that the best-laid schemes tend to go awry, it demonstrated it, by going awry. It’s kind of like onomatopoeia---you remember: the word used to identify the noise bees make actually sounds like the noise bees make. I think Burns was handing future generations a fresh figure of speech---let’s call it “gang-afting,” to add to metaphor, simile, alliteration, personification, etc. (If you’ve ever had to create a Match the Figure of Speech to its Name Quiz for an English class, you know that you can always use another figure of speech; if possible, you’d rather not resort to the use of Metonymy and Synecdoche). Burns gifted us with gang-afting: to start out with a good plan that gets royally messed up. I suspect there are other examples of it out there. We just didn’t know it because we weren’t looking for them, and we wouldn’t have known what to call them if we’d seen them. I could do a search of English literature and maybe come up with a few examples, but that would require time and effort, and alter my normal pattern. However, any reader who performs such a search will receive extra credit.
I’m more curious about how Burns determined that the best-laid schemes of mice go astray. We won’t give him any argument about men’s schemes, having watched a number of botched rocket launches (which were presumably planned by rocket scientists), but there seems to me to be no good reason to impugn the best-laid plans of mice. Granted, some schemes of mice go astray, unless those particular mice schemed to have their backs suddenly broken while they nibbled on a tiny glob of peanut butter or cheese. But I suspect that, among all the schemes those mice may have entertained for that particular evening---chewing into the box of rice in the pantry, eating the crumbs around the toaster, making the humans jump and scream, encouraging the Mrs. to try to make more mice---the plan to eat the food on that odd looking wooden contraption under the kitchen sink was not among the “best laid.” I would not want to be the reporter, interviewing the grieving mouse spouse on the following day, who suggested to her that the unfortunate events of the previous evening were the results of her hubby’s “best laid” scheme.