Barrel connectors

Photo of connectors
A 2.5 mm phone jack, with several different AC/DC barrel connectors

These things are a pet hate of mine for a variety of reasons:

The same connectors are used for wildly different voltages and polarities by different manufacturers, and connecting the wrong supply to the wrong device is frequently very destructive.  It's quite common for someone to try reversing the polarity if something doesn't appear to work, and they cause even more damage by doing so.

They come in a variety of sizes (lengths, widths, internal dimensions, retaining notches, etc), and it's not always obvious that you've mated the right pair together.  The tension of any sprung parts in the connector will sometimes make a sloppy-fitting pair work, but not reliably.  I've come across a pair that jammed together solid, because of poor manufacturing tolerances.  And if you're repairing equipment with a broken one, you sometimes find that they've used an unusual size that you can't buy a replacement for.

They're not robust.

They easily disconnect when you don't want them to.

If you're building equipment that uses them, consider using something else.  Or strongly consider including reversed voltage protection, and over-voltage protection into your equipment.  And label your equipment and supplies very well.