Here are few reasons getting a single system to work correctly for a few minutes in a university lab is much easier than getting thousands of systems to work correctly for months at a time in dozens of countries around the world.
1. You forgot to make your "unreachable" states transition to the initial (reset) state. Clock glitches, power surges, etc will occasionally cause your system to jump to a state that isn't defined or produce an illegal data value. When this happens, your design should reset itself, rather than crash or generatel illegal outputs.
2. You have internal registers that you can't access or test. If you can set a register you must have some way of reading the register from outside the chip.
3. Another chip controls your chip, and the other chip is buggy. All of your external control lines should be able to be disabled, so that you can isolate the source of problems.
4. Not enough decoupling capacitors on your board. The analog world is cruel and and unusual. Voltage spikes, current surges, crosstalk, etc can all corrupt the integrity of digital signals. Trying to save a few cents on decoupling capacitors can cause headaches and significant financial costs in the future.
5. You only tested your system in the lab, not in the real world. As a product, systems will need to run for months in the field, simulation and simple lab testing won't catch all of the weirdness of the real world.
6. You didn't adequately test the corner cases and boundary conditions. Every corner case is as important as the main case. Even if some weird event happens only once every six months, if you do not handle it correctly, the bug can still make your system unusable and unsellable.