NEW YORK ( TheStreet) -- Even as a U.S. debt downgrade, a worsening of the European crisis, spiking oil prices and natural disasters added unexpected twists for dealmakers in 2011, there's still plenty left to worry about in 2012. It's key to remember that in the volatile deals landscape there's still plenty of quick and sudden change that can occur in merger negotiations or hostile takeovers, which can have a major impact on share prices and earnings.
For instance, AT&T ( T) $39 billion merger with T-Mobile USA was set to be the largest deal of the year until an antitrust suit iced the tie-up in late December. That merger fail cost JPMorgan Chase ( JPM) its U.S. M&A advisory lead and AT&T a $4 billion breakup fee charge to its fourth quarter earnings. Overall, global M&A activity increased 3% to $2.8 trillion in 2011 with U.S. deals eclipsing the $1 trillion mark, a level not seen since 2007. In the U.S., healthcare, oil & gas, utilities, technology, real estate and telecoms were the top deal making sectors, respectively. Corporate divestitures were a bright spot for deal makers, rising 165% from 2011 and representing 5% of all deals activity, according to Dealogic. In the U.S., divestitures also didn't slow in the second half of the year, as companies spun assets to bolster shares, streamline their strategy and raise capital. For more on M&A trends and some investing ideas, see 10 Top Morningstar M&A Stock Picks for 2012. Even after an antitrust based-AT&T and T-Mobile USA deal breakup and a flurry of failed hostile takeovers that caused shares to gyrate in 2011, plenty of risk remains. Here's a look at five big 2011 M&A winners that could face challenges in 2012.