Why did my omelette have bubbles on top while cooking, develop tunnels, turn slightly grayish and have a rubbery texture? I rubbed olive oil on the pan surface first. I always cook omelettes the same way and this is the first time this happened.
Bubbles on top while cooking - If you beat air into your eggs before you pour, air would naturally come to the surface.
Develop tunnels - The only explanation I have for this is that you didn't beat the eggs enough, and the mixture was not homogenous. But based on the physical characteristic of bubbles, it would seem that you did beat the eggs, so I'm not entirely sure.
Grayish - Overcooked iron/sulfur compounds in the yolk. Overcooking usually involves heat, which is a chemical reaction from a more reduced state to a more oxidized state. This usually brings about a change in the transition state of an element (sulfur or iron). A change in transition state causes a color change.
Rubbery - Overcooked. The proteins have coagulated very tightly due to prolonged heat exposure.
When I cook omelettes I usually use Jacques Pepin's method, which gives a creamy inside. Youtube "Pepin omelettes," and it'll probably show up.