We may be talking at cross-purposes.
I don't think of "a mapping explosion" as a specific error or event.
I think of it as a general condition of having a lot of fields.
It's a condition that can lead to a number of problems (memory pressure, delays publishing cluster state..) and is the reason we introduced a soft-limit to the number of fields in mappings.
If there are 2000 fields in my mapping file but the number of documents I am indexing is low (50,000).
Sounds like a lot of fields for users to consider/search but shouldn't be too much of a problem.