As others have said, it could be a vacuum leak. But, here's another thing it could be:
I had this issue happen once on an older car where the car would literally start accelerate on its own. The RPMs would climb as though someone were pushing the gas pedal. What it ended up being is the vacuum controlled fuel pressure regulator had worn out and gasoline had started making its way into the vacuum lines and then into the air intake.
Just another thing to check for.
Snorkel Delete & AEM Drop-in