The most challenging one of these I ever found was to do with onions. What I like about it is that whilst it is contrived - these always are - it passes TenMinJoe's test. The task was set me upon hearing someone leaving my train saying 'You know, the onions onions onions one?'. Possibly the most frustrating conversational snippet ever. Obviously, I'd have to construct this sentence. Sleep nor food would be my solace until this was my defeated foe.

Obviously, I could have copped out. It would have been easy to have a couple both of whom were called Onions, who exclaim each other's names. It would have been easier to have a vegtable seller, declaiming his goods. Obviously, speech and proper nouns offer easy but ultimately unsatisfying recourse. Like cheating in Civ2. What I wanted was to find a sentence which made grammatical sense. Perhaps actual sense would be A Bridge too Far. I'd settle for some Colourless green ideas sleep furiously situation. It don't even occur to me that this could have been some -gry type puzzle. Credulous fool that I am.

I started with a simple sentence:
I bought some onions.
This was easy, I thought. I'm a third done already. This is great. Next step was obvious. Contrived, but evident.
I bought some, having called them onions, onions.
So there's two. Phew, I thought, I can do this. Ok, so I lied. I could have had 'seen his' over 'called them', but I prefered the silliness. Then I stalled. I could find no feasible way whatsoever to incorporate a further non-ejaculatory part of speech into the sentence. Damn that man!

Then, weeks later, trawling through C Literature from about 1880, I came across the phrase 'to know one's onions', apparently used to signify considerable familiarity with a topic. So. I was all set.

Incorporating my new friend was not difficult. The sense of the sentence is that, since I am such an expert on the things, I identified some objects as onions, and then bought them. The sentence in full runs like this:
I bought some, having called them -knowing my onions- onions, onions.
Oh yes. And now we can all get some rest.