Only a few times really, I do generally go by the 'work up to a dose' thing usually, and would never ask a family member for help...that's what BL is for!
I guess one time I was an ex's parents house standing in their garden, whilst they had builders renovating the garden. Thought it would be a really good idea to snort Ket on the down low in front of them and try and appear normal. Snorted a bit much and couldn't see in front of me / distinguish what the fuck was going on and just tried to remain as calm as possible knowing the duration was short enough that it should be fine soon, just don't start moaning in confusion and just keep looking at the floor or something.
And maybe when I last IV'd heroin, and felt it coming on really fast and stronger than usual and you know you have about 7 seconds before you pass out and hopefully don't fucking die. Not much you can do really...