If you do it on a large enough scale you'd get inflation as you have more money supply for the same amount of goods so that the money loses value. This would then lead the central bank to raise rates to counteract inflation. This in turn is bad for the economy (more expensive to take out a loan leads to less investment) so you could get a recession. I doubt that you'd be able to inject enough counterfeit money for that, though.

However, most central banks do that on a much larger scale at the moment to get that inflation and it doesn't really work (the Bank of England has stopped that though).