There's a child gate in their doorway, because his younger brother likes to run out of the room and get into trouble; their room is safe and so I let them watch a movie or play together in there sometimes. This only works when they're willing, of course.
Anyways, I told him I'd be right there. Then he says "I really need to go!" "Well, why didn't you tell me before it was urgent?!" "I really need to go!"
So I get up, and go over, and he's jumping and bouncing and shaking half-hysterically. "I need to go!" I look at him, and then take hold of him by the shoulders and stop the upper half of his body from moving, and tell him to look at me, and ask him again, "So why didn't you tell me before it got so bad?" "I really have to go!!!" *shake shake dance jump dance* "WHY - didn't - you - tell - me - before - it - got - so - bad?" I ask.
"I don't know! I really really have to go!" *jitter shake dance dance dance*
"What are you going to do next time?"
"I have to go!" *shudder dance shake shake bump grind*
"WHAT - are - you - going - to - do - next - time?"
"Tell you sooner!"
"Ok." So I lifted him over the gate and let him run into the bathroom. A minute later, he came out, and said "Daddy, my pants are wet."
"How did you get your pants wet?"
"Well, I went to go to the bathroom, and I dunno..."
Daddy 0, Kiddo 1.
(And yes, I'm sure it was at least partially deliberate, he was already in the bathroom, lights on, door closed, when it happened.)