Gil posted about the well-known psak of the MB that one may get a haircut today, because the combines joy of Shabbat and overrides the custom of not getting a haircut during Rosh ChodeshSefirat Ha-Omer. Personally, I follow the position of R' Lichtenstien and shave on Fridays during Sefirah so this doesn't affect me all that much, but in the velt, many people will be using this hetter, especially those with bad-looking beards who have no shot at winning the annual beard-growing contest (insert appropriate references to the relevant Cheers! episode here).
A Sephardic balabos asked if this works for him as well. So I started looking for others who relate to this issue, and simply couldn't find any. It wasn't the most comprehensive of searches, but there was nothing. Furthermore, the simple meaning of the Shulchan Arukh (493:3) itself would preclude this hetter. If there are exceptions to the rule that one may not get a haircut on Rosh Chodesh Iyar, especially if those exceptions occur every few years, we would expect the SA to qualify his statement.
The chiddush here is not that the Mishna Berurah manufactures minhagim. I posted a while ago on this issue (and had a great debate with bluke), particularly about the practice of saying the bracha before Kriat Shema together with the Shli'ach Tzibbur . The 'minhag' to lay tefillin without a bracha and take them off before Hallel on Chol Ha-mo'ed is another example that comes to mind. Today's example is a bit different in that it's a leniency and not a stringency.
Another example that comes to mind is the chumra of women not making kiddush for men. There are positions, such as the Shulkhan Arukh, Arukh Ha-shulkhan, and Gr"a, that hold that women may make kiddush for men. There are positions (Ba"ch, Prisha) which hold that they are not motzi men. MB creates this middle position whereby he paskens like the SA me-ikkar ha-din, but then says that a woman should not do so le-chatchila because of zila milta. All others who invoke zila milta do so to explain why it's ineffective. MB takes there logic and turns it, essentially, into an issue of tznius (See Shemirat Shabbat Ke-hilcheta on this issue), where it's muttar but you really shouldn't do it. This has become THE commonly accepted attitude toward women making kiddush.
I sincerely hope that the Mishna Berurah doesn't continue to dominate comtemporary halakhic practice, though, at least in the U.S.A., its hegemony is very strong. I've had American Talmidei Chakhamim, serious and learned men, tell me that the Mishna Berurah's psak has been accepted as binding, and that one may not follow an alternative position. Even the Kitzur Shulchan Arukh is being printed in editions which record instances where the MB disagrees. Soon enough, this will be all that people know.
Problem is, MB isn't really a halakhic work. It's a compendium and guide. R' Moshe Feinstein never quotes the MB as a halakhic precedent. It takes a snapshot of the halakhic process and presents a cautious conclusion. And it's not a lone voice either. There are alternatives to it, both Sephardic and Ashkenazic. I therefore have very mixed feelings about its hegemony over contemporary practice.