1. He apologised for a quarter of an hour for having asked which of his cousins did the cookery (ch. 13)
2. In chapter 15, when Jane introduced him to Mrs. Philips, he "[apologise] for his intrusion, without any previous acquaintance with her, which he could not help flattering himself, however, might be justified by his relationship to the young ladies who introduced him to her notice. Mrs. Philips was quite awed by such an excess of good breeding". He repeated his apologies when they leave the house, "and was assured with unwearying civility that they were perfectly needless".
3. In this chapter: "all Mr. Collins's scruples of leaving Mr. and Mrs. Bennet for a single evening during his visit were most steadily resisted", which may indicate that he was worried that they would be offended if he went, and apologise (maybe repeatedly) for that.