It's possible to fail on sort memory overflow even if there is an alternative plan that requires no sort.

With a collection that has two indexes like:
1) a_1_b_1_c_1
2) c_1_b_1_a_1

If you run a query like: {a: { "$in": [1]}, b:1, c: { "$in": [1,2,3]}}).sort({c:1}), a query shape will be cached with the associated plans that do no require sort and it is likely that the plan using the a_1_b_1_c_1 will get the top score.

If you now run query like: {a: { "$in": [1,2,3]}, b:1, c: { "$in": [1,2,3]}}).sort({c:1}), the cached plan with top score will be used. As the query requires SORT stage, the plan will be re-adjusted, however if at the sort stage we hit the 32Mb limit, the query will fail despite the fact that there is an alternative plan cached that uses the c_1_b_1_a_1 index, which if used, will require no sort and re-adjustment.