You have to worry about power dissipation as well as current. Here's what Gianluca has to say: "With 150mA of load at 12V the regulator will go in thermal shut down (or burn) because it will dissipate 1.3W instead of the 0.45W it could not exceed without a proper heat sink
At 12V it could easily provide 50mA (75mA @ 9V)
it is the same for the Mega2560"