The author of the piece I'm currently editing has a really annoying habit of omitting that whenever possible. He writes sentences like "Smalling adds Joseph Smith told them he had . . .". Sure, there's nothing grammatically wrong with it, because English allows omission of relative that in certain cases, but he does it a lot in cases that cause (temporary) misparsing. I've probably inserted one or two per page, and I'm on page 54 now.
The most annoying part of the piece, though, is the fact that the the body of the article is 38 pages, but the notes take up another 78 pages. I came across one note that went on for three and a half pages. Note to authors: if it's important enough to include in the article, put it in the body of the text where people will read it and where it will lend support to your thesis. If it's not that important, briefly mention the issue and then refer readers to some other sources that cover it in more detail. Heck, why not take all those tangential discussions in the endnotes and make separate pieces out of them if they're so important.