Jay Z did NOT pull a fast one on his audience in Greensboro, N.C., Sunday night ... when he brought a 12-year-old boy up on stage who stole the show -- it was all spontaneous.

We spoke with Angela, who took her 12-year-old son Justin to the concert in Charlotte Saturday.  She says Justin's dream was to "get noticed" by Hova and he was crushed when he was overlooked as he sat in the 8th row.

Angela says on the ride home they decided to give it one more try, and she bought tickets to the Greensboro concert the next day.  She says she was lucky and scored front row seats.

Angela tells us nothing was set up.  Justin jumped up with his sign and this time Jigga noticed.

She says after the concert Jay Z was true to his word and took them backstage.  They talked for 5 minutes about personal stuff.  No numbers were exchanged and Jay said nothing about signing the kid.

50 Cent thought twice about posting photos of his new baby boy on the Internet -- because moments after he loaded his Instagram with pics of his 16-month-old son ... he deleted them ... or tried to.

Fiddy posted the pics yesterday of Sire Jackson -- born 9/1/12 -- along with a bunch of funny captions. He even posted a side by side photo of Sire and himself to show the father-son resemblance.

For some reason, 50 then deleted the photos roughly an hour after posting them, but not before one of his followers saved them ... and leaked them back online.

As we reported, the rapper was charged with a series of crimes last fall -- including domestic violence and vandalism -- after a blowout fight with Sire's mom, during which he allegedly kicked her and destroyed a bunch of her property. The domestic violence charge was ultimately dropped.

Sire is Fiddy's second child. He also has a 16-year-old son named Marquise Jackson with another woman