And to my knowledge no bot has gotten past it or even bothered.
/s
Human captcha. :)
And if it’s something between friends why not use a private whatsapp group or IRC
But I could have came up with a challenge that anyone interested in our forum would have known. We were a rhythm games club, so a questions could have been "We organize a tournament for this game, in 3 letters". Anyone who heard of us would know it is DDR (it was our main event), and for those who didn't, all it took was a quick look at our website.
And even if it is just a hobby, you don't get the same thing from a forum compared to a WhatsApp or even a Facebook group. In fact, a few years after me and all the geeky admins left, they switched to Facebook, which completely killed the already dying online community.
Maybe nowadays we could use a Discord server, which does a bit more for a community than an WhatsApp group, and is more accessible than IRC, and saves history.
Another great thing about the forum is that I still have a backup from the time I still had admin rights, and I could bring it back to life on a private server with some friends, even though the club is now completely dead.
Memories are not lost, except for most of the external links (ImageShack...).
Heck, I've never heard of you lot but I immediately guessed DDR. Man I miss competing with randoms at the arcade in Brisbane city
All things must pass and all that.
In low scale settings, if it's a place almost nobody knows, then this works, but many approaches work like just asking "What is 1+1?". Scale is low, no bot writer will bother to adjust.
In high scale, high visibility, none of this works. The incentive to break your captcha is so high that you'd need to basically construct a reverse turing test. You need to assume that the attacker is very powerful and very smart and will spend months custom tuning their solution to vreak your captcha. This is really hard and how to do it is the interesting discussion.
In summay, the setting matters. If it's the first one, we can debate toys all day long but they only have entertainment value. If it's the second then this is really hard and state of the art is to click pictures with traffic lights.
When I managed a small phpbb forum, all I had to do was to change a few lines in the register page to make it non-standard and it stopped all bots. Better than the built-in CAPTCHA. Simply, no one cared enough about our forum to write a specialized tool, no matter how easy it was.
If it is all you have to protect, go ahead with your clever ideas, it can add a bit of flair to your website and stop bots effectively. For accessibility, you can always deal with special requests manually.
The problem is entirely different if you are Google. People will spend months trying to break your CAPTCHA for fun and profit. Hand crafted problems will be solved faster than they can be written so "bot vs bot" is essentially your only option.