I’ve had the whole tin cap come off the bottle when I’ve tried to unscrew Australian and Spanish wines. They’ve tasted fine, but is this a common problem?

—B.B., Potts Point, Sydney

Well, at least you managed to open the bottles. That’s another problem with screw-caps: sometimes they’re impossible to open at all – they just refuse to budge.

This situation usually results in me using a knife or the blade on a “waiter’s friend” corkscrew to cut through the line that’s supposed to break when you unscrew the cap – and, sometimes, there’s bloodshed.

Happily, it’s not a common problem, but I have witnessed desperate people puncturing the cap with a spike to get at the precious liquid inside.

Both problems have the same cause: the capping machine on the bottling line has failed to create the perforations in the cap that enable it to break apart when you apply counterclockwise pressure. Possibly, the capping machine needed to be adjusted. Perhaps the person supposed to be monitoring it ducked out for a smoko.

At least, in your case, the cap and its collar both came off easily, leaving the wine unaffected. This means the seal under the cap remained unbroken, and no air got in.

If you look at an unopened bottle with its screw-cap intact, you’ll notice a deep indentation that encircles the collar, immediately below the perforation. This keeps the collar of the cap attached to the bottle while you remove the cap. If the capping machine fails to make this indentation properly, the entire cap and collar can slip off.

Screw-caps are a great seal and, in my experience, rarely have a fault. In the rare instance they’re not perfect, we notice it. Corks are much more of a lottery.

Got a drinks question for Huon Hooke? thefullbottle@goodweekend.com.au