MH17 was the, "Wrong plane," says Ukrainian fighter pilot after shootdown.

[Major Nikon]

165. A little knowledge of aircraft performance also helps

The quoted service ceiling is based on relatively cool and optimum standard temperature and pressure readings. The warmer the temperature is and lower the pressure, the lower the service ceiling becomes on any given day. The accident happened in July during the warmest part of the day. I have no idea what the temperature was, but it's probably a safe bet that it was higher than standard.

The service ceiling is also not a hard limit to the aircraft and one that performs at book values (not all do) should be able to actually climb higher than the service ceiling. However, from a practical point of view, it's not that simple. The closer an aircraft gets to it's service ceiling, the harder it is to maintain altitude because the flight envelope gets very narrow. The chance of an engine(s) flameout is also higher. It can also take a very long time to get there because climb performance is diminished the higher you go. So the service ceiling and the highest an aircraft will typically go in practice can and usually is quite different.

Most aircraft aren't certified to carry max fuel when the rest of their load is maxed out. So it's entirely possible (and highly probable) that a hypothetically lightly armed aircraft is going to be carrying more fuel and will therefore be closer to its max gross weight than you might otherwise think. Fighter and intercept aircraft go through a lot of fuel, so it behooves them to carry as much as practical.