EXCLUSIVE

Rihanna just learned a hard lesson ... acting like an irresponsible Lindsay Lohan can cost you a LOT of money -- in this case $47,050.

TMZ broke the story ... Rihanna's ex-accountants have been trying to get the singer to sit for a deposition in a lawsuit she filed against them. Rihanna claims they mismanaged her money and owed her millions, but the accountants claim Rihanna's the one who's at fault.

According to the accountants -- Berdon LLP -- Rihanna's been blowing off deposition after deposition (Lindsay-style) costing them boatloads in attorneys' fees, so Berdon asked the judge to punish the singer.

And this month, the judge agreed, fining Rihanna $47,050 for Berdon's legal bills.

On top of the fine, the judge also ordered Rihanna to sit for a deposition once and for all on August 28 ... and if she doesn't show up, he might dismiss her case entirely.