The worst is when you have it plugged into a USB charger in the car on the way to a race to charge it, but it decides to turn on instead of charging and drain its entire battery, so instead of showing up with at least a 50% charged garmin like you left the house with, you have a dead garmin.
Always look to see if it's giving you the % charged signal. Otherwise ...
So this is a known issue, apparently? Seems like a software issue. Why doesn't Garmin send out a firmware update to fix it? I've had this happen several times over several years. If I put the watch on the charger, it should charge, no questions asked.
Anyway, my phone did okay. I'm planning a long adventure in a couple of weeks and might just use my phone because its battery lasts longer than the Garmin these days (or use both, and see which battery lasts longer …)
I've found the Garmin can only do one thing at a time. If it's downloading and you hook it up to charge at the same time, bad things happen. Let it finish whatever it's doing before you try something else. And if it's fully charged, take it off the charger and make sure it has shut off. Mine was down to 3% the other day after sitting on the charger all night, ceasing to charge but staying on.