I have some MOSFETs that turn on without any voltage applied to the gate. They turn on even without the gate connected to the circuit; I tested them in a simple switch circuit using an LED:
I also checked them with a multimeter. It gives a reading only between the source and the drain, the current flows only from the source to the drain.
These are the part numbers: k2519, k1985, k3115, 2d0n60f, and irf740. There is many others with the same problem. I have checked all of the datasheets, they say they are n-channel MOSFETs, so I don't know what the problem is.
Here you can see the LED is on without the gate even connected (the green wire). The orange wire is the drain, the blue wire is the source.