It happens from time to time, but normally in the way that after the MCB has cleared a fault that you can push the lever up, but it feels loose and the contacts inside stay open, or where its been exposed to sustained overloads, it'll weaken over time and might eventually trip when carrying a load less than In.
As to failing safe, well both the above failure modes fail safe, but as its a mechanical device there must be a risk (however small) that it'll jam, though with modern manufacturing methods its probably a small risk and this is backed up by the fact that there is no inservice test for them, the guidence is just to replace if they look worse for wear.
The risk is minimised by using recognised brands, rather than cheapo stuff that might be made in dveloping nations by companies who will fake technical standards approvals, though even a recognised brand is no guarretntee, sometimes even they can be subject to problems (think of the recent well known toy company inncident with lead based paints), and of course knock off products can enter the supply chain (cast your eye over them... if they look a tad suspect, then they might just be so!)
If a breaker does jam closed in a fault current situation though (quite unlikely), in a house, then the fuse provided by the electricity supply company should opperate before anything actually bursts into flame