Most microwave ovens have three safety microswitches in the door latch assembly. The diagram below is typical:

These microswitches aren't the most robust switches ever made and can be down right squirrely. They may test good one minute and then look bad the next. When a switch looks flakey like that, trust your meter and just replace it. Chief jumptrout's experience with this is typical:

Had a strange occurrence today on a microwave call.

The lower microswitch that activates closed when the door is closed was the problem.

With the switch in place and the door closed,there was no continuity in the switch.

I removed the switch and did a double test of the switch to verify a bad switch.

When the button was pressed,the switch made momentary continuity.

When completely depressed the switch broke continuity.

If you backed off on the pressure on the button,continuity resumed.

Has anyone seen a situation where this has happened?

Yep i have had that before a few times...nice catch. That can be easy to miss if your not diligent.

Source: Weird microswitch