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BEYONCE PISSED OVER LAURYN HILL DISS AT THE BET AWARDS!

Beyonce wasn't laughing when a boy band DISSED Lauryn Hill at last night's BET Awards ... in fact, Bey was so upset, she changed her acceptance speech (on the fly) to HONOR the former Fugee.

So let's start with The Diss -- Comedian Mike Epps and the group Mindless Behavior were co-presenting the award for Best R&B Artist when Mike jokingly tried to sing.

One of the members of the group quipped, "You sing BAD ... like Lauryn Hill's tax accountant bad."

The diss drew gasps from the crowd ... and pissed off Beyonce, who ultimately WON the award that was being presented.

B was so upset, she scrapped the speech she had prepared and defended Lauryn's honor instead ... saying, "I want to thank all the talented R&B women that came before me like Lauryn Hill and Mary J. Blige."

When B walked off stage, there was a woman waiting for her -- possibly a BET handler -- and B began to vent about the insult, saying the Lauryn Hill diss was "disrespectful" and "not cool."

The woman complimented Beyonce for adding Lauryn Hill's name to her acceptance speech ... to which Beyonce replied, "I just felt [the diss] was wrong."

Man! They can't even pee straight! Talkin' bout Lauryn Hill