Simply, a GPS approach, during which things get a little out of kilter, followed by the system's recognition of this, and loss of GPS navigation information...
Leaving you part-way down an approach, with no accurate navigation information...
All the RAIM messages in the world won't help you find your way out of trouble, when you have no tracking aids available, nor the current ability to interpret them...
My post was perhaps by way of an attempt to counter IO540's remarks, which seemed to perpetuate the current myth that transport aircraft navigate almost exclusively by computer - jurassic navigation aids such as ADF/NDB still have a part to play, albeit very seldom.
I might add, that if technology provided all the answers, we would be flying a lot more MLS approaches than we are!
(edited to add a new thought - there's honesty!!!)