If you never made a mistake, you probably didn’t do anything.
A lot of paragraphs are in italic. Some should be, but why are all the subsequent paragraphs also in italic? The first few paragraphs are in roman, but not the others. I study the CSS. Italic is only where it should be. I can’t find it in inheritance. I turn text wrapping off and check that every paragraph that’s supposed to have italic has a proper start tag and a proper end tag. They do. But one of those paragraphs must be causing everything afterwards to be italic without inheritance. This is taking me two days to analyze. I forget about using W3C’s validator, because, while it works well, I don’t want to run everything through it all the time when I should be getting it right by hand. Finally, I comment out some of the paragraphs that are not supposed to be italic but are. I comment out about half and find which half is causing the problem. Then I comment out about half of that bad half to narrow it down. I keep shrinking the commented-out area till I find it. It was stupid. It was the paragraph that was supposed to have “sic” in it. But, in my HTML, instead of writing “<em>sic</em>”, I had written “<em>sic</sic>”.