LED glows slightly during soldering
Although LEDs ought to include a reverse diode for ESD protection, the heater coil induces a magnetic field into the high impedance LED with enough mutual coupling to generate xxx µA of dim current.

Anecdotal

I sold over 1 million 5 mm LEDs, and it wasn’t until I flew down to this one client in New Zealand and saw their poor handling and soldering practices that I changed the design of the LED to include ESD protection and a dozen recommendations to improve their volume manufacturing process. Then their 0.5% failure rate dropped to near zero.

Although the current may be low, the reverse voltage may exceed -5&nbsp;V and partially wound the junction.