This is a screenshot of a saved apex class. 



The bare bones essentials: 

public without sharing class WebOrderMethods {

    public static Map<String,String> webOrderMap = new Map<String,String>();
    public static List<Map<String,String>> lineItemListMap = new List<Map<String,String>>();
    
    public static Web_Order__c buildWebOrder() {
        Web_Order__c thisOrder = new Web_Order__c();

        if(webOrderMap.size() > 0){
            for(String key : webOrderMap.keySet()){
                String value = webOrderMap.get(key);
                
                if(key == null || value == null){
                    continue;
                }
    
                switch on key {
                    when 'Email' {
                        thisOrder.Contact_Email__c = value;
                    }
                    when 'Notes' {
                        if(thisOrder.Notes == null){
                            thisOrder.Notes__c = value;
                        }
                        else{
                            thisOrder.Notes__c += '\r\n'+value;
                        }
                    }
                    when 'ShipmentMethod' {
                        thisOrder.Shipment_Method__c = value;
                    }
                }
            }
        }

        return thisOrder;
    }
}

I found an issue with my code in which I was referencing a custom field improperly. Here I'm iterating through a list of my custom object records "Web_Order__c", and I noticed I forgot to include the "__c" affix. 

Just to test, I tried to query this field thru Anon Apex and got the expected error:

Why did this save properly if there's no such field?