NEW YORK ( Stockpickr) -- Over the course of the summer, the U.S. economy has looked weaker and weaker with each passing economic report.At this point, economists have been forced to sharply lower their forecasts for the remainder of 2012, and some are suggesting that we may end up back in recession as we head into 2013, especially if the much-discussed "fiscal cliff" comes to pass. Thankfully, most major companies used the last economic crisis to get in better shape, cutting costs and raising cash. But some companies just never learn. They muddle along with high levels of debt and are simply unprepared to handle a slumping economy. >>5 Toxic Stocks You Need to Sell Now We're not talking about little mom-and-pop companies, either. A handful of major corporations, residing in the S&P 500, are quite vulnerable to a tougher climate. They carry lots of debt or must pay high levels of interest on their bonds every quarter. If cash flow shrinks or even turns negative, those weak balance sheets could turn into time bombs. Here are five stocks that look especially vulnerable.