Are you, at least in a professional capacity, occasionally asked to provide rather ad hoc comments on this or that web page markup? I am. Regularly, though, I make the mistake of assuming that the developers who will be receiving my comments are aware of web standards and accessibility concerns that I typically consider part of good development.
This is a mistake because there is an entire range of differently skilled and concerned developers out there. Some are full-time developers who should really know better, but don't. Others simply find themselves having to markup web content, in which case I don't think we can really expect them to be fully aware of and up-to-date with best practices.
In either case, I'm finding that it's worth it to take the few extra minutes to follow through in my comments with detailed explanations of every step involved and the reasons behind my suggestions. When I forego this step, and assume that the developer will, on their own, get the picture and follow through in implementing best practices to their proper end, I too regularly find myself getting frustrated and later having to provide a second set of comments that more clearly provide the detailed instructions and explanations that I reasonably could and should have offered the first time around.
Granted, this situation may just be up to laziness on my part, or it may be that old saying about what happens when you assume coming to fruition, but from now on, when asked for these one-off and informal types of reviews, I'm going to try to offer detailed explanations of the steps to take from beginning to end, and not just assume that identifying the problem and providing general instructions will do. For instance, when I suggest that headings on a page are not properly nested, I'll take the extra minute or two to identify what heading should be at what level, and why this matters.
Not only does this capitalise on a potential learning moment for the developer, and, I think, show some additional professionalism and concern for doing a good job, but it will, often enough I expect, save me from doing more work later.