On the "Bills & Income" tab, "Projected Balances" sub-tab, if an account went into a negative balance within the display period, Quicken used to show a warning icon at the bottom (legend) to the left of the account name. Recently it stopped showing that, and now just shows the normal colored circle. This is especially bad when an account is overdrawn by only a few dollars, since it may not be obvious that the balance has gone below the zero line.
Quicken Classic Business & Personal Version R63.21 Build 27.1.63.21 on Windows 10 Home