502 Error on SharePoint Site

My clients and I received a 502 error on our SharePoint Site, then the SharePoint Site came back up by it self. I have 1 SharePoint App server and 2 SharePoint Front End servers, I checked the Event Viewer and noticed event id: 3355 and event ID 6398 on all 3 servers around the time that we received the 502 error. My question is: Can 1 or both of these event IDs cause the 502 error and if so how would/should I go about making sure that this will not happen again?