Volatile stock markets, recession fears and the US-China trade fight have traders on edge. Now, they are grappling with another problem: garbled data.

The troubles started Monday with a glitch on a key market-data feed operated by the New York Stock Exchange. That was followed by what looked like calamitous drops in Vanguard Group mutual funds, incorrect prices for stocks like Apple Inc. being flashed over Bloomberg terminals and the Dow Jones Industrial Average suffering a mysterious outage.