I have a fixture with 4 MR16 (two pin) halogen bulbs. If I replace 2 of the halogen bulbs with LED's all bulbs work. If I replace 3 halogen bulbs with LED's all the bulbs act like a strobe light (not flickering because it is a constant on/off pattern). If I replace all halogen bulbs with LED's none of the bulbs work.
The fixture does not have a dimmer.