Fork left in microwave, hole in microwave wall
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.