When importing the following file using mongoimport:

{ _id : 1, x : { $maxKey : 1 } }
{ _id : 2, x : { $minKey : 1 } }

mongoimport does not appear to be processing the $maxKey and $minKey extended JSON syntax correctly.

It's hard to tell using the shell:

> db.test.find()
{ "_id" : 1, "x" : { "$maxKey" : 1 } }
{ "_id" : 2, "x" : { "$minKey" : 1 } }
>

but the x values are actually nested documents, and not MaxKey and MinKey respectively.