`spriteframe_t` is defined as

  typedef struct
  {
    // If false use 0 for any position.
    // Note: as eight entries are available,
    //  we might as well insert the same name eight times.
    boolean rotate;

    // Lump to use for view angles 0-7.
    short lump[8];

    // Flip bit (1 = flip) to use for view angles 0-7.
    byte flip[8];
    
  } spriteframe_t;

which is okay to splat with all-ones as long as `boolean` is either a typedef for a fundamental type(*) or an enum – because C enums are just ints in a trenchcoat and have no forbidden bit patterns! The C99 `bool`/`_Bool` is, AFAICS, the first type in C that has fewer values than possible bit patterns.

So yeah, on C99 and C++ this always had UB and could've broken at any time – though I presume compiler devs were not particularly eager to make it ill-behaved just because. But in pre-C99 it's entirely fine, and `rotate == true || rotate == false` could easily be false without UB.

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(*) other than `char` for which setting the MSB is… not UB but also not the best idea in general.