I normally get away with reusing the crush washer by turning it over. If I notice a little leakage then I change it. Definitely a cross threaded drain plug can become a serious nightmare. I just did the rear diff about 6 months ago and I just noticed what looks like the rear seal of my transfer case is leaking a little bit so that might be next on the list. At 215k miles it could be time to do so.
I had a place forget to put my oil cap back on a long time ago. Let’s just say it was a huge mess. My little Geo Prism puked oil everywhere. Lol