I think Alex was referring to internal consistency within the router (between linecards), not external consistency. For example, if linecard X believes that a packet should be forwarded to linecard Y, but linecard Y's forwarding table is older than X's, Y could misforward the packet, causing a forwarding loop or a dropped packet. Thus, it can be the case that neither the old path nor the new path is taken.
Yes, there are ways to approach this problem, but it is a problem that central-forwarding systems will not have.
It's a problem that few distributed-forwarding implementations have. Doing a full destination-IP lookup on the incoming and outgoing linecard is wasteful. In most implementations, the incoming line card determines the outgoing interface and neighbor and informs the destination linecard of that. The destination line card then sends it out to whomever the source line card specified, without actually doing a lookup in it's FIB. Consistency is still a problem, particular the case of line cards being inconsistent with the central processor for non-transient periods of time. (As in, they're out of sync, but nothing knows it and no update is in process, so they are just going to be out of sync forever.) -- Brett