I left a fork in my microwave and there were no sparks but there is now a small hole on the inside wall. The microwave works fine but is there anything dangerous about this?
First off, what is the hole in? Is it just a hole in the plastic interior, or is a hole clean through the metal shielding? If you're confident it's just the plastic, that should be fine - that's not part of what's keeping it safe.
If it's actually a hole through the metal, then the power transmitted is roughly proportional to (d/λ)^4, that is, the fourth power of the ratio of diameter to wavelength. (See this question on physics, with more details found on another question there. That proportionality is from a simplified version of the problem, but it should be an okay approximation.)
With a 12.2cm microwave wavelength, using a 12.2cm hole as the baseline, 0.01% as much leaks through a 1.2cm hole, 0.1% through a 2.1cm hole, 1% through a 3.8cm hole, and 10% though a 6.7cm hole. And the full power of the microwave is 750-1500W, so the absolute worst case power leaked through those holes is .15W, 1.5W, 15W, and 150W. So for holes less than 1cm (leaking at most .15W) you should not have to worry. A 2cm hole (leaking at most 1.5W) also would likely be safe, since the energy shouldn't be too focused, but that's the range where I'd start getting nervous.
The wavelength of the microwaves in the oven is roughly 12 centimeters or 4.7 inches. Therefore, for any of the microwaves to escape the interior of the oven, the hole would have to be 4.7 inches in diameter. If the hole is smaller than this, then the waves won’t leak. If the hole is larger, microwaves might leak. However, even if the waves leak out, it is highly unlikely that they will cause any damage, as microwaves are non-ionizing in nature and do not carry much energy as individual waves in open space. If they escape, the microwaves would dissipate (or die out) quickly in air, not causing any harm, and likely going unnoticed...
One thing that can also happen is that this setup will cause interference to radio communications (WIFI, HAM radio...) or even damage someone's radio. While not a direct threat of injury, if you interrupt somebody's business with interference from a known defective device, they can hold you liable for damages.