Compilation of the following fragment shader fails:

uniform vec4 vecX, vecY;
uniform int iter;

void main() {
    for(;;) {
        bool b = (dot(gl_TexCoord[0], gl_TexCoord[0]) < 4.0);
        gl_FragColor.z = iter == 0 ? vecX.z - vecY.z : 15.0;
        if(b)
            break;
        gl_FragColor.x = 1.0;
    }
}

r300: Initial fragment program
FRAG
DCL IN[0], GENERIC[0], PERSPECTIVE
DCL OUT[0], COLOR
DCL CONST[0..2]
DCL TEMP[0..2]
IMM FLT32 {    4.0000,     0.0000,    15.0000,     1.0000}
  0: BGNLOOP :16
  1:   DP4 TEMP[0].x, IN[0], IN[0]
  2:   SLT TEMP[1].x, TEMP[0].xxxx, IMM[0].xxxx
  3:   MOV TEMP[0].x, TEMP[1].xxxx
  4:   SEQ TEMP[1].x, CONST[0].xxxx, IMM[0].yyyy
  5:   IF TEMP[1].xxxx :8
  6:     ADD TEMP[1].x, CONST[2].zzzz, -CONST[1].zzzz
  7:     MOV TEMP[2].x, TEMP[1].xxxx
  8:   ELSE :10
  9:     MOV TEMP[2].x, IMM[0].zzzz
 10:   ENDIF
 11:   MOV OUT[0].z, TEMP[2].xxxx
 12:   IF TEMP[0].xxxx :14
 13:     BRK
 14:   ENDIF
 15:   MOV OUT[0].x, IMM[0].wwww
 16: ENDLOOP :0
 17: END

Another example that fails is (maybe more common):

uniform vec4 vecX, vecY;
uniform int iter;

void main() {
    for(;;) {
        gl_FragColor.z = vecX.z - vecY.z;
        if(dot(gl_TexCoord[0], gl_TexCoord[0]) < 4.0 && iter > 15)
            break;
        gl_FragColor.x = 1.0;
    }
}

r300: Initial fragment program
FRAG
DCL IN[0], GENERIC[0], PERSPECTIVE
DCL OUT[0], COLOR
DCL CONST[0..2]
DCL TEMP[0..1]
IMM FLT32 {    4.0000,    15.0000,     0.0000,     1.0000}
  0: BGNLOOP :15
  1:   ADD TEMP[0].x, CONST[2].zzzz, -CONST[1].zzzz
  2:   MOV OUT[0].z, TEMP[0].xxxx

  3:   DP4 TEMP[0].x, IN[0], IN[0]
  4:   SLT TEMP[1].x, TEMP[0].xxxx, IMM[0].xxxx
  5:   IF TEMP[1].xxxx :8
  6:     SGT TEMP[0].x, CONST[0].xxxx, IMM[0].yyyy
  7:     MOV TEMP[1].x, TEMP[0].xxxx
  8:   ELSE :10
  9:     MOV TEMP[1].x, IMM[0].zzzz
 10:   ENDIF

 11:   IF TEMP[1].xxxx :13
 12:     BRK
 13:   ENDIF
 14:   MOV OUT[0].x, IMM[0].wwww
 15: ENDLOOP :0
 16: END

The driver could cope with that by inserting something like
     CMP x, -x, 1, 0
before the IF.