Answer:
But one incident have just ruined the plan, had not been anticipated.
Step-by-step explanation:
To maintain the coherence and cohesion of the sentence, the expression "been anticipated" was added to explain the expression "had not" presented in the sentence previously. These two expressions are related to the incident that thwarted the narrator's family's vacation plans. In this case, the expressions show that the incident was unexpected and happened without being detected or predicted by any member of the family, however, the incident was serious enough to end the vacation.