Inside there is a large capacitor that can retain a substantial charge (read lethal) even after the microwave has been turned off for hours. Touch the wrong place by accident and it can be Hello Benazir.
The manufacturers give the warning for that reason and also because if somebody is ham fisted in reassembling the microwave cooker, it could result in a radiation leak.
Service agents are equipped with the safety and test equipment to avoid accidents and some manufacturers use special bulbs that are not generally available either.
Of course there are always ways and means, but if you are not 110% aware of what you are doing, then do make sure that your life assurance is up to date.
Many Microwaves require the casing to be removed when changing the bulb. If so there are a few issues. Firstly there will be mains around so unplug the thing first. Secondly the Magnetron capacitor may still have a very high charge at very high voltage. There is a bleed resistor which discharges this over a few minutes but they have been known to fail and there is no obvious indication if they have. Simplest precaution is to unplug the microwave the night before doing the repair and don't plug it back in until you have changed the bulb. Finally make sure the casing is fitted correctly when you replace it and put all the screws back.
Ours has a bleed resistor across it. Of course, whether it still works or not...
Apart from that, yes, like many bits of equipment there are hazards in opening them and if you're not sure what you're doing then it's probably best if you don't. That said it wouldn't (hasn't) ever stop me. Our DVD has just been resurrected despite "DO NOT OPEN" on the back.
Back on topic, "not suitable for microwave cooking" has never deterred me trying.
I think the vegetable spring rolls on Boxing Day were about the worst failure; I've never actually managed to *burn* anything in a microwave before now...
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