Or sometimes the technology is there, but we're just not ready. For example, a very rudimentary steam engine existed in ancient Rome, and was described by others as early as 30BC. While it was highly unlikely to happen because the infrastructure/materials were limited, imagine where we'd be now if the Industrial Revolution had happened in Ancient Rome.
It also makes me wonder if there's anything around today that's considered some oddball quirk of physics that might change the world in the future when we're ready to use it.