Incident one is a lack of attention to detail, and possibly failure to use a checklist. When you say you left the "oil cap open," do you mean the actual filler cap on the engine, or the access hatch on the cowling? Oil filler cap off can lead to oil loss and ultimately engine failure, and I've seen a few pilots return with windscreens covered in oil. Access hatch leads to embarrassment, and I've seen a few ripped off in the slipstream. Expensive, but not dangerous. Both, however, indicate a degree of carelessness, and it doesn't take a lot to be a real problem on a flight. Failure to open an oxygen valve, switch a fuel selector, or other seemingly innocuous things have brought aircraft down and killed people.

That said, if it was an oil access door, failing a checkride is a bit excessive.

Stalling the airplane while slipping or skidding is an invitation to depart controlled flight, particularly if you're not aware that you're approaching a critical angle of attack (or low airspeed).

The issue of your "clearance" by the examiner, and an actual clearance by an air traffic control facility is a bit more of a gray area. You could have asked the examiner if if wanted you to continue out straight ahead, but you acted with an actual communication from ATC, and I don't think you could be faulted for that. In my opinion, if the situation was as you described, the examiner was out of line.

Forgettting to retract flaps speaks to procedures, flows, and checklist use, but really shouldn't be a show-stopper on a checkride. Embarrassment, but if that was all, then a reasonable examiner might have pressed on. If you flew around for twenty minutes, unaware that your flaps were extended, that might be a different matter. Aviation is about details; it only takes a single missed detail to cause undesirable results.

60 days without a checkride should have been 60 days of preparation for the next training or checking event. It's hard to look back at 60 days that coiuld have been used to prepare, if you dind't take advantage. Don't make that mistake again.

When an examiner fails a student on a checkride for something like an open access door or a missed checklist item, it's usually not just that single item that causes the pink slip. It's usually the overall impression that the examiner gets. Can I sign this person off and will they be safe? Would I put my family in the airplane with this person? If the answer is no, or even "not sure," then the latitude granted by that examiner is not likely to be very wide.