Almost every day that I update the portfolio, I see swings in prices over 20% in both directions. When reviewing those equities, they always have zeros for high, low and volume. Usually the erroneous value is repeated over time, creating spikes in the price history. These incorrect values are creating large swings in the overall portfolio value that do not in any way correspond to the market moves.
I do not have "real time quotes" selected in my preferences. Most of the time, the updates are run after market hours.
Here's an example (one of many):