I have an MK consumer unit with the house ring main wired into a MK 5932s MCB ( https://www.mkelectric.com/en-gb/Products/CP/sentry/MCBsinglepole/MCBsinglepoletypeb/Pages/5932s.aspx )
Occasionally when switching the MCB into the off position the switch will move toward the off position but it seems to stop half way between the two. While in this state power is still flowing to the sockets.
If I give the MCB switch a hard push down and then push it fully up again I find that it does not always reset to the on position rather moves back inbetween the on and off positions. This whole time the power remains on.
After some attempts it will reset correctly.
Initially I assumed it was a defect with the MCB but have confirmed that this isn't the case by switching the MCB over for a new one.