"A code P0011 refers to the VVT (variable valve timing) or VCT (variable camshaft timing) components and the car's PCM (powertrain control module, also called an ECM). That consists of a few different components but the P0011 DTC specifically refers to the camshaft (cam) timing. In this case, if the cam timing is above a set limit (over-advanced), the engine light will be illluminated and the code will be set. The "A" camshaft is either the intake, left, or front camshaft."
Most likely a P0011 DTC will result in one of the following: hard starting, poor idle, and/or stalling. There are potentially other symptoms as well. Of course, when trouble codes are set, the MIL (malfunction indicator lamp, a.k.a. the check engine light) illuminates.
A P0011 DTC trouble code may be caused by one or more of the following:
Incorrect camshaft timing
Wiring problems (harness/wiring) in intake timing control valve control solenoid system
Continuous oil flow to VCT piston chamber
Failed timing valve control solenoid (stuck open)"
It could be the CVVT oil control valve assembly needs cleaning. See http://www.hmaservice.com/files/331/5002805/09-EM-002.pdf. It doesn't list the 2.7 liter in the Tucson, but does list the same engine in the Santa Fe. The P0011 error code is one of those sporatic issues that can apparently be triggered by this problem.
Define "serious"? The engine wasn't designed to run with a P0011 error code. At the very least, I would think power and mileage would be diminished. Long term implications? I'm not sure. Can't be good for the pollution control systems like the catalytic convertor if the engine is constantly running too lean or too rich because of the wrong timing. I beleive the TSB states the situation is covered by warranty. Could also be caused by one of the problems cited by jsinton. I would get it looked at if the vehicle were mine.