Out on a limb and On the spot

On the spot and out on a limb are semantically related in in trouble topic. In some cases you can use "On the spot" instead the phrase "Out on a limb" as an adverb or a noun or an adjective.

On the spot adverb - At the place in question; there.

Out on a limb and on the spot are semantically related in dangerous topic. You can use "Out on a limb" instead the phrase "On the spot" as a noun or an adjective or an adverb, if it concerns topics such as danger, vulnerable, in a jam, open to attack.