The biggest trouble sign for stocks may be bonds.
High-yield bonds, specifically, often are seen as an effective proxy for movements in the equity market. If that's the case, trends in junk are pointing to a rocky road ahead.
Average yields for low-rated companies have jumped to 7.3 percent and spreads between such debt and comparable duration Treasurys have widened dramatically, according to David Rosenberg, chief economist and strategist at Gluskin Sheff.
History suggests that fallout in stocks is not far behind.
"If you think the equity market is heading for a spot of trouble here, the high-yield bond market is having a coronary," Rosenberg said in his daily market analysis Thursday.