My worst parenting fail came during one of ds1's 'Terrible Two' tantrums - it's a story I have shared here, a number of times - mainly to reassure other mums of toddlers that they are doing just fine, because I have done far worse than them.
The previous night, ds1 had had a huge tantrum, lasting over an hour and a half - I can't remember why now (he is nearly 26, so I think I can be excused). That morning, it was not yet 9am, and he'd already been screaming for 45 minutes, and I was on the phone to dh, in tears, saying I couldn't cope.
Suddenly it went quiet, so put the phone down and went to investigate - silence, in toddlers, is always a Bad Sign. Ds1 was standing by the full kitchen bin, eating the leftover Doritos that dh and I had thrown in there last night, when we didn't finish them all.
That's not the worst part. The worst part is that I stood there and had a debate with myself over whether to stop ds1 enjoying his tasty bin snack - I knew that, if I stopped him, he would start tantrumming again, and I just couldn't face it, so I let him carry on eating until he'd had enough and wandered off to play - then I emptied the bin and found a new home for it, in a cupboard, so he couldn't go back for seconds.