This is the safety line. A small leak at the mouthpiece that you can clean and monitor is one thing. But there are red flags that mean stop using the device now:

Liquid in the battery compartment. If you see e-liquid near the battery or it feels wet when you open the battery door, do not charge it. That's a fire risk. Stop using it and contact the manufacturer.

Overheating. If the device gets hot to the touch during normal use, that's a sign something's wrong. Stop using it.

Persistent leaks after fixes. If you've cleaned and re-seated everything and it still leaks, don't keep using it in hopes it'll stop. It won't.

In those cases, stop using the device, contact the manufacturer or your retailer, and don't try to charge a liquid-compromised device.