An overdose of any cardiac medication could cause cardiac arrest. Digoxin, Epinephrine, Norepinephrine, Dopamine, Dobutamine. Any of those, given at a large enough dose "fast push" could do it.
The above mentioned air embolism and potassium chloride are also good choices.
Any electrolyte abnormality could cause it. Specifically, hyper- or hypokalemia (high or low potassium), hypoglycemia (low sugar, could be caused by an overdose of insulin), hypomagnesemia (low magnesium can cause arrhythmias), hyper- or hypocalcemia (calcium abnormalities).
If the person is in a coma because of head trauma, brain herniation from swelling could cause sudden cardiac arrest.
Fluid around the heart (pericardial tamponade) could cause it. Things that cause PT are cancer, kidney failure, and trauma.
HTH