There are a handful of passages from the original Frazer translation of Apollodorus that have been withheld from our version due to their context being unclear (i.e. block quotations which interrupt narrative flow). They are as follows:
Epitome 1.1.21: There is a sizeable portion here on the marriage of Pirithous and the fight of Theseus with the centaurs that has been removed. Frazer's footnote on this section comments that:
"This passage concerning the fight of Theseus with the centaurs at the marriage of Pirithous does not occur in our text of Apollodorus, but is conjecturally restored to it from Zenobius, or rather from his interpolator, who frequently quotes passages of Apollodorus without acknowledgement. The restoration was first proposed by professor C. Robert before the discovery of the Epitome; and it is adopted by R. Wagner in his edition of Apollodorus."
I checked the Perseus version of this section and the way that they have handled it is to include the omitted section as a quotation from Zenobius, complete with Frazer's explanation above.
Epitome 1.6.15a, b and c: These sections are quoted paragraphs from another work, Tzetzes's Scholia on Lycophron. Frazer includes in his footnotes for this section that:
"The following three paragraphs are extracted from the Scholia on Lycophron of Tzetzes, who seems to have borrowed them from Apollodorus."
Perseus has handled this by having separate tags for sections 15a, 15b and 15c in which they present this material as a quotation, with Frazer's citations, but as distinct from the original narrative.
Ostensibly though it is from other works, the truncated material is attributed to Apollodorus. The missing material in 1.1.21 does actually add useful context to that passage, but for the sake of clarity it may be best to find an unobtrusive way of presenting these as quotations belonging to other authors who may in turn have borrowed them from Apollodorus.