GLSL vertex shader compile sometimes fails

I've been getting intermittent errors compiling a vertex shader, preparing for the first render of a newly created OpenGL context. It's the same vertex shader that usually works, on the same hardware. After glGetShaderiv detects the failure, the info log usually shows this:

Vertex shader failed to compile with the following errors:

Yes, that's the whole thing.  The video card is ATI FireGL V7700, and the GL_VERSION string is 3.3.11554 Compatibility Profile Context FireGL.  Any clues?

More info here: http://stackoverflow.com/questions/22109033/intermittent-glsl-vertex-shader-compile-errors