Weather: thunderstorms, turbulence, icing, wind shear, cross/tailwind, very low/high temperatures.
ATC: traffic congestion, TCAS RA/TA, ATC command, ATC error, ATC language difficulty, ATC non-standard phraseology, ATC runway change, ATIS communication, units of measurement (QFE/meters).
Airport: contaminated/short runway; contaminated taxiway, lack of/confusing/faded signage/markings, birds, aids U/S, complex surface navigation procedures, airport constructions.
Terrain: High ground, slope, lack of references, "black hole".
Other: similar call-signs.