There was a legit problem with them at the start, but that was ages ago. The Vac 700 and the first 5xx-series would crack regularly. I think they've got that handled these days. I have a whole bunch and no problems at all. They're some of my most reliable pens.

I'm just going to sneak in here to say... maybe? but we have three in our house, and the two little vacs have both cracked. Not catastrophically, but weirdly. And I can't comment on my kiddos, but I'm someone who is *very* mindful about over tightening. 🤷‍♀️ Still useable, still enjoy them.

The only recently made TWSBI I've had crack was a Swipe. I made the obvious error (at least in hind sight) of thinking the Swipe's clip could handle fabric between it and the cap. Clipping in and out of a pen roll just cracked and popped the finial end of the cap right off.