TOM TOM RED X APPEARS AND UNIT WON'T SHUT OFF

With the TomTom GPS units, if you use it infrequently, you may often find the unit has a dead battery when you go to use it.
This is because turning the Tom Tom off normally with the power button doesn't actually turn it off fully, but instead puts it in a standby mode.
Even though the screen is off and it appears to be shut down, it's still using some power to maintain configuration settings.

If you know you won't be using it for a long period of time, you can press the reset button on the bottom of the unit (this can be done while it's off), and it should put it in shutdown mode, where the battery will not be drained so rapidly. If you do this, you can confirm it's in the shutdown mode by tapping the on button. This normally will power up the Tom Tom, but in the shutdown mode, it will not do anything. To wake it up from the shutdown mode, press and hold the power button for a few seconds, at which point it should turn on and greet you with the Tom Tom drums. It takes quite a while to boot up from this mode, which is probably why they defaulted to the suspend mode instead of a true "OFF" state.

For some of the newer units, they've omitted the reset button, so the process is a little different. Instead of pressing the reset button, do the following:
with the unit on, press and hold the power button. The shutdown screen should appear, but continue holding down the power button for another 12 seconds after the screen turns off. Timing is a bit touchy, because if you hold it too long, it'll end up just rebooting, and then you'll have to try again.
If you're successful in the shutdown, you can test it by briefly pressing the power button. It shouldn't turn back on with just a short press. If it comes on with the tap on the power button, you'll have to try again.

This whole process is a bit of a hassle, and extends the turn on time greatly, so you'll want to plan ahead when doing this, and just shut off normally if you expect you'll be using it within a couple days. However, the long boot up is preferable to having a completely dead unit when you need it, especially if you're away from a power plug...