Speaker cable has a bigger conductor that's meant to handle the amount of power being pushed through an amp. Instrument cable is shielded and has a much smaller conductor, which can melt or separate if used between a head and cab. If that happens it might cause some damage to the amp as well, since it will be operating under no load (or a short, depending on how the cable breaks).

Do not run an instrument cable between your head and cabinet. You (or your amp) will die.
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Speaker cables use a thicker wire and are designed to take the outputs of amps. Using an instrument cable to connect an amp and cab will fry the cable and eventually your amp. Using a speaker cable for your guitar will likely sound choked out, but I doubt it would break anything...
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