One thing that i see often: just because both your debug cable and your target runs at the same voltage (eg: 3.3V), doesn't mean it's safe to plug them together. You could easily have a situation where your target or your debugger is off, so at that point you're powering the off thing through the uart pins (the esd diode on the pin).
The safest thing is to have a 4th vref/vccio pin, then the debugger should power its tx signal from that pin, at whatever voltage it's at. Same for the RX pin, it's not nice to have a pullup to a certain voltage when the target is off.
The safest thing is to have a 4th vref/vccio pin, then the debugger should power its tx signal from that pin, at whatever voltage it's at. Same for the RX pin, it's not nice to have a pullup to a certain voltage when the target is off.