How did I know?!
Today I inserted my 10 x 2 hour ticket with six empty slots and got the dreaded "re-try" message. Tried it in a second machine, same result. So I've gone over to the customer service guys and they've had look at it, opened the barrier and let me through. I was heading down the stairs and suddenly thought "hang on a minute..."
I've gone back and pointed out to them that I didn't actually have a validated ticket - slight hint of irony there! So they tried it, same result. The woman then scraped it with something, they tried it, and it now worked. One validated ticket.
Unbelievably, we pulled into Ripponlea and there were the ticket inspectors. I don't think I've ever seen them there before, yet by the biggest coincidence I was within a whisker of getting caught for no ticket!
No doubt they would've had no problem accepting my story and letting me off, had I not clicked back at Flinders Street...!