The U.S. Federal Reserve has joined its counterparts around the world to keep interest rates low and liquidity high through programs including quantitative easing . However, S&P warned that unforeseen events could have consequences.

Essentially, how manageable all that debt is will come down to financial conditions, which have been easy for most of the period since the global financial crisis thanks to central bank accommodation.

"In recent years, credit conditions have been largely favorable, and corporate issuers have actively issued record levels of debt, much of which has been used to refinance existing debt, lower funding costs, and extend maturities," the agency said in a report. "Funding conditions began tightening last year as falling commodity prices and volatile equity markets contributed to investor unease and a flight to quality."

That's how much of the $51 trillion in global company IOUs is maturing between now and 2021, according to data from S&P Global Ratings, which warns of potential dangers ahead.

The corporate debt pile is continuing to pile up, with a $10 trillion bill coming due over the next several years.

"Commodity-producing sectors and countries have experienced a sharp decline in credit quality over the last year, which has contributed to episodes of tightened credit conditions, particularly for commodity-related companies," the agency said. "We expect that uncertainty in the global financial markets will be heightened following the U.K.'s vote to leave the EU (Brexit), which could provide headwinds for financing conditions through 2017."

In its forecast of the most likely scenario, S&P said, "Credit conditions and issuance levels should be sufficient for companies to manage global corporate rated debt maturities through 2021."

However, the looming issues with corporate debt are beginning to gather attention on Wall Street.

The amount of investment-grade debt rated BBB has climbed to 44 percent of issuance, according to David Rosenberg, chief economist and strategist at Gluskin Sheff. That classification is the second-lowest rating for investment-grade debt; that level compares to 10 percent about 40 years ago, Rosenberg said.

"Credit quality is becoming an issue after the last few years of debt issuance used to fund share buybacks, dividend payouts and M&A activity," Rosenberg said in a note earlier this week.

"The question is whether the 50-plus companies that Moody's downgraded to 'junk' status in the first quarter was just the thin edge of the wedge," he added. "The remainder that are a mere notch away have a combined debt load of $294 billion — which would just flood the high-yield marketplace."



Of the total $10.3 trillion coming due, 26 percent is speculative grade, or junk.

One of the problems from the corporate debt perspective is that most of the $1.9 trillion or so of corporate cash floating around is held by only the biggest companies. Overall, U.S. companies are holding the lowest level of cash to debt in a decade.

In an earlier report, S&P projected that corporate debt issuance would balloon to $75 trillion by 2020. The agency warned of a possible "Crexit" should conditions tighten and bond market liquidity evaporate.