i.e. can a bug that existed early in the toolchain history propagate in the binaries but not the sources?

c_i is the i'th version of the compiler.

    c1 c2 // c1 compiles c2

    c2 c3 // c2 compiles c3

Source code of c1 has bug, source code of c2 and c3 is fixed. Bug in c1 causes binary of c2 to have bug. Binary bug in c2 causes binary bug in c3.

You can still, if you really want to, build the Go 1.4 from its C source, then compile the go compiler with that result.