I changed the word "you" to "your" by inserting an "r," in [[Gerrit/Tutorial]], but the edit summary said I inserted 10 characters, and the [https://www.mediawiki.org/w/index.php?title=Gerrit/Tutorial&diff=prev&oldid=732497 diff report] showed about 15 lines, with various things shifted around. I was using Chrom on Windows 8.
I reported this on the support page, and 126.96.36.19917:17, July 13, 2013 reverted my edit, and suggested I file a bug report. [https://www.mediawiki.org/wiki/Thread:Project:Support_desk/Extensive_diff_report. Support desk resolution.]
I have also been noticing that when a change comes up on my Wikipedia watch list, I sometimes have difficulty telling what an editor did, because the diff report is full of unmodified content.