The second was when I was doing routine maintenance on another chamber & noticed that the liquid-line sight glass was bubbling. Must be low on gas thought I, so I started to fill with R502. The sight glass didn't clear, so I kept filling. Bearing in mind this was a bench-top chamber ten minutes passed before there was an almighty sound of escaping gas & the whole lab filled with oil vapour. My mentor later told me it had a blocked drier.
One thing: watch both gauges while charging.