First, Mr. and Mrs. Loisel assumed that the necklace was made of real diamonds. This made them to go into severe debt to purchase a real diamond necklace, which led them to years of hard work and poverty.
Second, Mr. and Mrs. Loisel assumed that they could not simply tell Madame Forestier the truth, which that the necklace she had given Mrs. Loisel had been lost. This assumption, too, led to the consequences of poverty and hard work.
Third, Mrs. Loisel assumed that it was necessary for her to appear to be wealthier than she really was to attend the event in question, and Mr. Loisel, while he tried to persuade her otherwise, went along with this assumption. Had she not felt the need to borrow what she thought was an expensive piece of jewelry, the story would have had a very different outcome.
In general, it was the snobbery and dishonesty of both that give us this wonderful story, a true lesson on what not to do.