Last weekend I felt sorry for Victoria Beckham. Just for a moment I felt like I'd been there. That's not a sensation I get very often where she's concerned. We have very different lives, I suppose. I've never been in a girl band, she's never been on I'm Sorry I Haven't a Clue. She designs her own clothes, I barely wash mine. She's married to David Beckham, I'm not. (I have now run out of facts about Victoria Beckham.) Anyway, we're different people.

One of the few things we have in common, other than endoskeletons, is being on Twitter. Last weekend I read an article headlined: "The aliens are coming, tweets moonstruck Posh", which approvingly recounted the Twittersphere's disdainful reaction to her tweeting: "UFO hovering above our house last night!!!!!" accompanied by a photograph depicting what most people would agree is a full moon.

"Dear oh dear, what a moron!" is the tone of the article. It quotes one tweeter as saying: "Someone want to tell Victoria Beckham that 'UFO' is the moon?" Now there's a phrase steeped in swaggering scorn. I can only imagine it being said in an estate agent's voice followed by "What are you driving at the moment?" I looked it up and found the tweet was from @Smudge1208, a "Huge Girls Aloud and Formula One Fan". (I've heard of Formula One but I'm not familiar with the work of Huge Girls Aloud.)

I found lots of similar responses. From the warm: "just a full moon Vic!!......x"; through the concerned: "Victoria, to me it looks like the moon"; past the condescending: "never saw the moon befor?" [sic] and the mocking: "thinking the moon was a u.f.o is the funniest thing ever #sillycow"; to the slightly off-message: "your fucking disgusting david doesnt love you" [sic(k)].

They had a field day at her expense. Many of them, I'm sure, thought they were very clever. What they were all too stupid to realise is that Victoria Beckham was obviously joking. She didn't think the moon-like image was actually a flying saucer. She just thought it would be funny to say so. I don't necessarily agree with her, but I appreciate the instinct for levity. And unfunny though the joke arguably is, it's not easily confused with a serious assertion.

I've often experienced this kind of obtuseness on Twitter. Apart from the steady stream of people who, when I say I've written in the Observer, seek to correct me that in fact it's the Guardian (as if they would know better than I do which newspaper I write for), any joke will get several smart-aleck responses from thick-alecks who haven't got it.

Just last week, when sending a link to the online version of my column, I wrote: "My Observer column this week is about my aversion to getting anything done. So it's miraculous that I wrote a column." Not hilarious, I know. But I hope you can detect the humorous attempt. Someone replied with the comment: "Oxymoron much?" I nearly screamed. I prefer it when they call me a c***. I mean, no, it's not strictly an oxymoron but basically that's the very point I was clearly making myself.

What baffles, infuriates and depresses me is that each haughty response to Beckham's tweet is the product of a separate person going through the same series of terrible pieces of reasoning, at any stage of which they could have abandoned the idea of their fatuous comment.

Victoria Beckham does not have a great reputation as an intellectual. She doesn't come across, to the extent that we can garner anything about what she's actually like, as the sharpest knife in the drawer. This leads many people to assume she's unintelligent; but I think it unlikely. There's plenty that suggests the contrary: her fairly wide-ranging career, continued success, wealth and circle of friends all imply that she's at least averagely bright. The fact that she looks so serious in photos is more likely to be because she thinks that's an expression that flatters her face than because she's genuinely consumed with muddle-headed solemnity. But, even if you think she's a bit dim, there's no evidence that she's so mentally subnormal as to mistake the moon for a spaceship. Thinking she might be is the first wrong conclusion in this series of boneheadedness.

The second is the respondents' belief that, if Victoria Beckham thought she'd spotted an alien spacecraft, her only reaction would be to tweet about it – and to wait until the next day to do so. Those five exclamation marks on Twitter provide scant compensation for the disgraceful lack of urgency that course of action would betray. Again, you may think she's stupid, but surely not that stupid.

The third link in the chain is that, having come to such an unlikely conclusion, the tweeters are sufficiently sure of it to put it in writing. They don't merely suspect that Beckham is guilty of this implausible level of idiocy – they're 100% convinced. This is puzzling because, to have made the first two misjudgments, they must be very foolish people, or at least very lazy thinkers, who, you'd imagine, would have little experience of being right. They must surely be in almost permanent possession of the wrong end of the stick in every aspect of their lives. You'd think they'd have become more tentative about expressing their opinions. But no, it turns out they are so thick that they've failed to notice, throughout their long history of ridiculous blunders, their own denseness.

And the delicious pudding to this tasting menu of unreason is that it's not the moon. They're even wrong about that. There was no full moon in Los Angeles that night. All the people calling her an idiot for apparently not knowing when she's looking at the moon, don't know when they're not looking at the moon. Don't worry though – I doubt this means it was a spaceship (although, strictly speaking, it is currently an unidentified flying object – another gold star to VB). It was probably a helicopter searchlight shining towards the camera. The helicopter noises may have been partly what inspired Beckham's quip. Someone want to tell @Smudge1208 that the moon doesn't make a noise?

I like Victoria Beckham for making a joke on Twitter. Being jolly and jokey with strangers is a generous thing. And what was her reward? It made some idiots feel clever. That's what led to the banking crisis.