It must be contagious. Marianne is so disgusted with the vulgar Mrs. Jennings and her prying, her thoughtless, interminable raillery, and her lack of tact, and she makes no secret of her disdain for the insufferable neighbours at Barton Park. And yet, in Ch. 18 what does fair Marianne do? She blurts out her notice of Edward's ring and succeeds in embarrassing him and causing him pain; and, had it not been for Elinor's conviction that the hair was her own, and her determination not to regard it as an affront, she might have been pained likewise. Oh, very well done, Marianne! Well done, indeed!
OK, so she at least apprehends her blunder and is mortified, and perhaps in her case it derives from her overly developed romantic sensibilities rather than vulgarity, but a blunder it is nonetheless. How shall she censure Mrs. Jennings and Sir John now? I seem to recall a certain phrase about stones and glass houses... ;o)