Leaving Maker's Mark bourbon (or any other whiskey) in the fridge overnight with the cap off and only a little bit left in the bottom of the bottle should not cause any significant issues or spoil the bourbon. Whiskey is a distilled spirit with a high alcohol content, which acts as a natural preservative, preventing it from easily spoiling or going bad.
However, a few things might happen when you leave bourbon exposed to air and temperature changes:
Evaporation: With the cap off, some of the alcohol may evaporate over time, which could slightly change the whiskey's flavor and aroma. This evaporation might be more noticeable if the bottle is almost empty, as there will be more air in the bottle relative to the remaining liquid.
Temperature Effects: Storing whiskey in the refrigerator might dull the flavors temporarily, as cold temperatures can numb the taste buds and mask some of the more delicate notes. However, when you take it out and let it warm to room temperature, the flavors should return.
Oxidation: Oxygen exposure can potentially lead to minor changes in the whiskey's profile over an extended period. For instance, it might soften some of the harsher alcohol notes, but it could also mellow some of the whiskey's distinct flavors.
Overall, leaving a little bit of Maker's Mark bourbon with the cap off in the fridge overnight should not ruin the entire bottle or make it unsafe to drink. It might undergo some slight changes in taste and aroma due to the factors mentioned above, but it's unlikely to spoil or become undrinkable. However, to best preserve the flavor and quality of your bourbon, it's generally recommended to keep the bottle sealed tightly and store it in a cool, dark place.