Yes, it's another 'is this a RIDDOR' question, though it's an academic query because it didn't occur to one of our people, nor involve our equipment.

If a piece of roped-access equipment does not operate as it should with the result that it releases a rope when it should not have released, and a person drops a short distance (being caught by their second rope), would you regard that as a RIDDOR reportable incident?

The regs say (item 1 in schedule 2) "The collapse, overturning or failure of any load-bearing part of any lifting equipment, other than an accessory for lifting."

In isolation, I'd interpret 'failure' as including failure to operate correctly, but...

INDG453 says to refer to the list online at http://www.hse.gov.uk/riddor/ and that says (actually at http://www.hse.gov.uk/ri...dangerous-occurences.htm ) "Failure in this context refers to components which suffer mechanical breakdown during the normal operation of the lifting equipment", but in this case nothing broke, it just didn't behave as it was supposed to behave.

Any comments?

For me I look at the intent of this section of the regulations. Simply it is there to identify latent defects in plant and equipment, thus enabling the HSE and industry to take action to prevent a catastrophic incident. Simply trend analysis. I know that doesn't answer your point directly.
If it was a one off then I would not report if nothing failed permanently. If the item is as new and you can replicate the defect then I would report as it may have an impact on other users.
Personally I would be going to the manufacturer ASAP for the answer / reporting it to ensure they are aware and are able to investigate and inform customers if their is a defect.