Hardware specifics: NVidia
These are known issues for current Nvidia hardware/drivers.
GLSL shader conformance
Early Nvidia GLSL implementations are known to be quite accepting of features of other shading languages like Cg. This has lead to problems when developers using Nvidia cards unknowingly use these features which cause shader compile errors on other hardware.
The classic problem case is using vec3(1,1,1) instead of vec3(1.0, 1.0, 1.0) as GLSL (as of version 1.1) does not do automatic type promotion.
Newer drivers from Nvidia (80+ series) seem to emit warnings when features are used that are non-conformant. However, developers are recommended to run their shaders through the 3DLabs shader validator or make the validator part of their development toolchain.
These issues may affect older drivers/hardware