Knowing when to shut up and actually doing it. I have a lot of time for this hard-to-learn skill! 

As a child, I was the only one of my Mother's four children who could cause riots with their mouth. At 7 or 8 I was once left at the dinner table with my older (19 years my senior) brother's girlfriend and decided that that scenario would be peak timing to tell her that "my Mummy thinks you're a witch". Not sure "witch" was the right word, but it's a great memory. If 'putting your foot in it' was a GCSE, I'd have easily got an A*. I was truly talented in that arena. 

I grew out of that quite quickly (thankfully) but I really feel as though I've recently mastered the ability to keep my mouth fully shut and at the right times. I've certainly become a million times more considered in my responses. If communicating during difficult circumstances, I've learned to be careful and measured in what I say and it's a skill that I really value now that I'm slowly cruising towards the end of my twenties.