Possible Error Sources
Things like thermal buckling, atmospheric effects, etc. will be discussed here.
NOTE: As efforts to catch up from page 1 of the presently 147 page thread progress, items may shift from Open to Closed and thus at this time may be incomplete or incorrectly categorized.
Effects caused by interaction with the air
It can be shown that forces generated by interaction with the air at normal atmospheric pressure can easily be as large, or larger than the claimed forces generated by the EM drive. Consider that the higher power devices are reaching temperatures of 80 degree C or more within the space of a few seconds, and the air inside the device could be rapidly heated to much higher temperatures if it has any significant moisture content.
Air currents - external - Air currents present in the test environment, inadvertently generated by the experimental sequence, or even mundane things such as sunlight or air conditioning.
Thermal Effects - General Atmospheric - Heating of the surroundings of the drive, causing air currents which generate forces on the drive.
Thermal Effects - Hot air balloon - Heating of the air inside the drive causes lift. The lift varies with orientation, depending upon where venting holes are located on the drive.
Thermal Effects - Expansion - Heating of the air inside the drive causes it to expand and push out of holes in the drive, until the drive reaches equilibrium temperature.
Thermal Effects - Jet - Heating of the air inside the drive pushes it out. Fresh air is drawn in to replace it, resulting in a continuous jet, like a microwave driven pulse jet.
Helmholtz Resonator - Air inside the device is vibrated, and causes continuous thrust by expelling and drawing in air in the manner of a Helmholtz resonator. 
Crookes Radiometer - Thermal differences across the device cause a flow of air over the device, which causes rotation in the manner of Crookes Radiometer.  Crookes radiometer shows that even operating in a partial vacuum, air flow from small thermal differences can cause significantly more force than a photon rocket of similar power.
Magnetic and Electric interactions with the surroundings
Induction of magnetic or electric fields in the apparatus and surroundings, causing repulsion/attraction with surroundings.
Induced magnetic fields pushing against the Earth's magnetic field.
Other mundane explanations
Thermal Effects - Drive - Heating of the structure or components of the drive causes the drive to change shape, altering its centre of gravity
Vibration of the device (Dean Drive Effects) when power is applied causing a ratchet-type rotation effect, due to differences in the static friction.
Magnetic or electrical effects causing the device to alter shape and weight distribution.
Ablation of the surface of the device. E.g. copper atoms from the frustum accelerate in one direction and their reaction force causes a small acceleration on the device in the opposite direction.
Surface Effect. Radio waves travel on very shallow surface. Copper oxides [black] and Copper Sulphates [green] Increase Surface resistance. Lowers Q and thrust. These can be removed with Acid washes and rinsing.
Photon Rocket Interpretation - The theory that in some way the EM Drive is simply a strange Photon Rocket.