If everyone chooses a completely disparate sets of validators the network will be unlikely to reach consensus that a particular version of the ledger is the one true and accurate ledger. But, in practice, people's UNL lists will overlap. This overlap causes the honest validators to come to the same consensus.
Every honest user of the system wants the system to reach a consensus. Validators will choose which other validators they trust specifically because they also wish to reach a consensus. Essentially, all honest users of the system cooperate to ensure a consensus is reached and maintained. And, of course, a lack of consensus is easily detectable.
To achieve unique node list:
The validators run Ripple nodes which process validations on their behalf. The list of validators that we trust not to collude to defraud us is called our Unique Node List (UNL).
For stronger assurance that something is true, we add more validators to our UNL. For example, we could add Itchy and Scratchy. Who are also not very trustworthy, but are also unlikely to collude to defraud us.