I encountered an issue where the geocoder was throwing some address geocoding anywhere but even close to the address.
The example is 1920 Martha Drive, East Helena MT 59635
If you run this code, https://www.mapquestapi.com/geocoding/v1/address?key=KEY&inFormat=kvp&ou..., the geocoder places the coordinates in Las Cruces NM, not East Helena MT.
I'm guessing that the geocoder must be looking for the closest possible match to the street. I think a better fallback would be if there isn't a full address match, then fallback to geocoding on the city, state, and zip.
I wrote some code to implement a fallback myself, but it was frustrating to say the least. MapQuest isn't the only geocoder that did this. Another gave coordinates for Ireland and Mexico, so at least MapQuest was in the correct country.