757 Driver.
FlCh Trap
I fly the 777 and from other comments here it would appear this is not a factor on the 737. On the 777 however it is possible to find oneself without the speed protection you might imagine whilst decending using Flight Level Change.
If you were to select a lower alt and activate FlCh the thrust will (usually) come back to idle and the FD will command an idle power descent at the selected speed.
Were you to fly the aircraft manually and ignore the FD commends, for example by reducing the descent rate, to avoid clioud perhaps, then the speed will reduce.
In most modes on the 777 with A/T engaged the thrust will maintain amber line, or 'wake up' just below amber line and increase speed to top of amber band.
In FlCH the A/T will do nothing untill the MCP alt has been reached, the speed could decay to stall speed.
This is why it is vital to either follow the FD or if it is not programmed correctly turn off the FD which will then give speed protection if the A/T is armed.