I'd had my old briefcase, which my dad gave me, for maybe 20 years. Unfortunately, though, one of the feet on the bottom got bashed off and I had to cover it in duct tape, and the spring in one of the locks went so I had to open it manually every time. I decided that its time had come, so bought a new one.
The new one isn't as heavy or as rigid as the old one (which was more of an attaché case), and it's only 3cm wider than the maximum allowed for Ryanair cabin baggage. When I bought it, the woman in the shop had a hard time getting the catch to work, but I figured it would ease up after a while so I purchased it anyway (hey, there was a sale on, a third off!).
When I transferred everything from my old briefcase to the new one yesterday, I had a look at the catch. It was indeed a bit stiff. I wondered if locking it and unlocking it might make a difference, so took the key, turned it ... and unlocked it.
Hmm. Apparently, it had been locked the whole time and the shop assistant had been opening it and closing it regardless.
Anyway, it works like a dream now. I don't think I'll bother locking it again, though. This photo of the key shows why.
I've had lockable diaries with more sophisticated keys than that.