Right now everything appears to be fine. All nodes have full reachability. But failure of a single component will introduce reachability problems that shouldn't be allowed to happen.
I've deliberately drawn the Ethernet L2 topologies as lines (rather than switches) because it's an L3 problem. Assume that the L2 gear is super-robust: multiple power supplies, multiple supervisors, etc... The muliaccess Ethernets might even be 10BASE5 :-)
The problem is not a typo (if there are typos, I'll fix 'em). The problem is also not related to failure of a non-redundant device like R5. If R5 goes down, all of the hosts in Area 5 will become unreachable. That's obvious. The problem I've introduced will be somewhat surprising because it's in a redundant area. It's subtle, but detailed in the diagram, and can be eliminated with a telnet client. Can you spot it?