Despite all of the attention on Berkshire Hathaway's ( BRK.B) portfolio this week, the stock itself is showing some cracks. That's because Berkshire is forming the exact opposite trading pattern from the one in Polaris right now. Here's why buyers should beware. >>5 Rocket Stocks for Another Week of New Highs Berkshire is currently forming a descending triangle pattern, the bearish opposite of the ascending triangle in PII. The descending triangle is formed by downtrending resistance pushing down from above shares and horizontal support to the downside, in this case at $111. A breakdown through $111 is Berkshire's sell signal. A parabolic drop in relative strength since June indicates that Berkshire has been woefully underperforming the S&P 500 in recent months -- you can see it from the stock's inability to make new highs in November. Buyers look anemic here, but failure to catch a bid at $111 is the start of something bigger. Don't sell unless Warren Buffett's baby unless shares fall through that price floor.