Plot hole: When Bond and Tibbett break into the underground lab and find the device which drugs the racehorses, you can see that it has a large light emitting diode (LED) on the top of it. What possible use would that be, since the device is implanted in the horse's leg, and is supposed to be covert? (00:37:20)
Plot hole: After sneaking into Zorin's secret laboratory, why did Bond and Tibbett decide to spend another day there? They would have known that the two guards they fought would inform Zorin they were spies and blow their cover. Besides, they already knew how Zorin was winning the races, so staying another day was just suicide. (00:39:40)
Chosen answer: Zorin is a psychopath. He may have meant it when he told May Day he loved her, or he may have just been telling her what she wanted to hear. Either way, when he thinks she'll drown in the mine as well, he doesn't consider it worth scrapping the plan just to save her. Once she escapes and has the bomb, he looks shocked partially because she wasn't killed but mostly because she's chosen to sacrifice herself to thwart his plans, something that a self-interested psychotic personality can't comprehend.
Captain Defenestrator