12/4 10:40 AM PT -- The move cost Beverley a nice chunk of change ... 'cause he just got slapped with a $25,000 fine for "throwing the game ball at a spectator," the NBA announced Tuesday.

L.A. Clippers player Patrick Beverley fired a basketball at a courtside heckler Sunday night after the fan allegedly said, "F*ck your mother" ... the 2nd major incident between a player and a Mavs fan in 2 weeks.

The 30-year-old guard had reportedly been trading verbal jabs with Don Knobler -- a local celeb in Dallas -- throughout the Clippers' game on Sunday ... when things reached a boiling point in the 4th quarter.

So, what set Beverley off? Don't talk about his mother.

Beverley grabbed the ball and threw it at Knobler -- who calmly collected it after it struck him in the leg. Beverley was ejected.

After the game, Beverley told reporters Knobler said, "F*ck your mother" twice.

Knobler later admitted to talking about Patrick's mom but denies using the f-bomb in his trash talk.

Seems Mavs fans have really been getting to opponents lately -- it was just Nov. 17 when Kevin Durant cussed out a different courtside fan who had been running his mouth.

Durant was fined $25,000 for the incident. He later bashed the heckler as "corny and weak."

In both instances, Durant and Beverley alerted refs and security about the heckling fans ... but ultimately took matters into their own hands.