Mistaking a temporary jump for a sustained bull market can be costly.

In 41 so-called bear market rallies since 1928 — gains of more than 10 percent that are later wiped out — equities fell an average 25 percent after peaking. The surge between March 9 and April 9 ranks as the steepest 23-day advance since 1933, according to data from Howard Silverblatt, an S&P analyst based in New York.