Certain uses of DISTINCT_SCAN plans clash to the same plan cache key between each other and/or other similar find queries. 
Notable cases include:

1. distinct('x', filter) and find(filter) and vice versa
2. distinct('x', filter) and distinct('y', filter) with indexes x_1 and y_1. 

Use of a cached plan with the same planCacheKey leads to inefficient execution, but correct results.