It all depends on the microwave oven. The majority of ovens open with a mechanical lever (in many cases, can be also be opened by hand by grabbing the edge of the door) which will trigger a sensor that shuts down the emitter while the door is opening. Depending on how fast the door was opened, microwaves may still be bouncing around and could leak out.
OTH some ovens, like the more modern builtin cabinet microwaves, have an electronic button that shuts down the emitter before triggering the door latch/spring to open.
In any case, the duration and the amount of energy released during the opening motion vs. emission lag will probably not be enough to significantly harm you as dielectric processes need to establish a field to polarize molecules that will heat up as consequence.
Not enough to harm you, but how about a nearby device with an antenna?
Would a microwave burst e.g. fry the speaker of a crystal radio (where the antenna bridges nearly directly to said speaker) which happened to be tuned to a radio-band frequency for which 2.4GHz is a harmonic?
Or, for regulated products like 2.4GHz wi-fi routers, does FCC “accepts interference” testing take microwave bursts into account, or are we slowly degrading some component in them every time we open microwave ovens near them?
Anecdata, but for 2 years I have had a Chromecast Audio and a cheap Creative 2.1 system about 20cm away from my microwave, which is the type where you can just open the door to stop it. No problems so far.
Don't microwave ovens in large part depend on standing waves (resonant with the chamber size) to deliver the power levels they do?
Directed microwave energy can ruin electrical equipment nearby. You can find videos of this on YouTube, usually out of Russia.
Whether it'd do the same with a very short, unfocused momentary burst from reflection inside the oven cavity depends on your equipment I guess. I wouldn't leave my hackrf with RX amp turned on nearby, anyway.
OTH some ovens, like the more modern builtin cabinet microwaves, have an electronic button that shuts down the emitter before triggering the door latch/spring to open.
In any case, the duration and the amount of energy released during the opening motion vs. emission lag will probably not be enough to significantly harm you as dielectric processes need to establish a field to polarize molecules that will heat up as consequence.