The message
from Rod contains these words:

Tim Decker wrote:
Has anyone else been having problems with opening tins? We found it
really hard to open tins with a previous tin opener. So I got an old
fashioned butterfly one and this didn't work very well, so bought
another from Asda (35p) and found this didn't work. Finally went to
John Lewis and paid for an expensive one (£7) and have now found this
doesn't work properly.

This made me think that maybe the manufacture of tinned goods has
changed slightly.

We have a Culinare SafetyCan 2 opener. Has an interestingly slightly
different mechanism - works OK. Not noticed anything very different
recently. Mind, the rims of cans seem a bit neater/thinner than they
used to be - are they welded rather than soldered these days?

Many (but not all) cans are now made so that you're expected to to make
a horizontal cut just under the rim, rather than a vertical cut inside
the rim.

Buy an appropriate design of modern opener, make sure it's a quality one
and use it in accordance with the design of the can.