Heather Graham -- NYC Pad Catches On Fire ... Stupid Candles!

FIRE SAFETY 101 ... never leave burning candles unattended -- a lesson Heather Graham learned the hard way ... WHEN HER BATHROOM WENT UP IN FLAMES!!!

Heather's swanky NYC pad caught fire Saturday night ... after she left a few mood-setting candles burning in her bathroom before heading out for the night. The flames later ignited some clothes hanging nearby ... according to the NY POST.

The NY Fire Department raced to the scene -- and spent 45 minutes battling the blaze, says the report.

Heather -- who was not home at the time -- was quickly informed of the situation and rushed back ... only to find her 12th-story-crib a little toastier than she left it.