• Weather: 14 degs F, 30.62 in (15.039 psi) atmospheric pressure, 88% relative humidity.  
• Vehicle and modifications: 2013 STi Sedan, COBB Catted Turboback Exhaust, COBB SF Intake/Airbox, COBB 1000cc Fuel Injectors, DW65c Fuel Pump, COBB/LPC FPR Upgrade Kit (last three modifications do not impact boost levels observed; they only serve to provide adequate fueling during the scenario).

• A 3-port boost control solenoid
• Tuning changes

Boost creep is dangerous because:

• It can exceed the factory fuel system's capacity, meaning that dangerously-lean air-fuel ratios and engine damage can result.
• Excessive boost can cause detonation that cannot be effectively controlled with the factory ECU's normal response methods (reduced timing).
• If extreme detonation has been observed, from poor fuel quality or excessive boost, for example, one of the factory ECU's methods for protecting the engine is to lower boost to the mechanical minimum.  When the system exhibits excessive boost creep, this is no longer possible and engine damage is likely.

Boost creep is undesirable from a tuning perspective because:

• The calibrator has little/no authority to control boost levels throughout a large part of the run.
• Highly variable boost response depending on environmental conditions as system transitions below/above wastegate flow limit thresholds.
• When the wastegate's flow limit has been reached, exhaust gas back pressure will undesirably rise (inefficient, causes detonation threshold to rise).

Boost creep is most likely with:

• Low ambient air temperatures and high atmospheric pressures (low-lying NW and NE regions are most affected, see purple within marked areas of US elevation map). (http://weather.unisys.com/usgs/3sec/index.php)
• Many airflow modifications, such as upgraded turboback exhaust, upgraded intake, upgraded turbo inlet, etc.