An inflation bug, for instance, can go undetected In monero, and it's possible that's already happened.
It's more than possible; it's very likely. Imagine spending a month of full-time work to find an inflation bug, but the bounty is not worth it. You'd be tempted to quietly inflate the supply of Monero in order to compensate yourself for the time you spent working. And then you'd disclose the bug to prevent someone else from finding it and making Monero worthless, which would prevent you from exploiting future inflation bugs. So there is a very clear risk/reward ratio for finding and subtly exploiting inflation bugs in Monero.
What's more, Monero developers, notably FluffyPony, say that you shouldn't hold XMR -- that you should only hold it long enough to buy something. This is the opposite of what most other cryptocurrency developers say, which is to buy and hold. I have to wonder if some of the Monero developers are subtly trying to keep people from getting wrecked in the event that some random person finds an inflation bug and dumps on all the exchanges.