The explain command is willing to accept unrecognized arguments:

> db.c.drop()
true
> db.runCommand({explain: {find: "c"}, unknownArg: "unknown"})
{
	"queryPlanner" : {
           ...
	},
	"serverInfo" : {
            ...
	},
	"ok" : 1
}

We should consider changing it to reject unfamiliar arguments with a clear error message, returning ErrorCodes::InvalidOptions. One danger of this loose validation is that a future version of the server will introduce a new option. The fact that an older version is willing to ignore this new option, rather than failing on an option it does not understand, could complicate the upgrade procedure.