This is exactly why I always use glass to send beer out, even though I would prefer to use plastic. It's so frustrating trying to fill a plastic one and the trauma is exactly how you describe!the rubber bung you press the bottle against is too small for PET so to provide any kind of seal you have to press the bottle hard up against the filler and hold it there. The slightest movement let’s the pressure out and you lose control of the fill.