scadnano allows "paranemic" crossovers (https://doi.org/10.1021/ja038381e, https://doi.org/10.1021/acs.chemrev.8b00207), i.e., crossovers that connect two domains in the same direction:

Exporting to cadnano (Export --> cadnano v2 from menu) and then reimporting that same .json file (File --> Import cadnano v2 from menu) gives an error:

The first domain mentioned in the error exists, and is the 3'-most domain of the scaffold. But the second domain (helix=2, start=15, end=16) does not exist in the original design.

The scadnano file with this design is

{
  "version": "0.16.1",
  "grid": "square",
  "helices": [
    {"grid_position": [0, 0], "max_offset": 17},
    {"grid_position": [0, 1], "max_offset": 17},
    {"grid_position": [0, 2], "max_offset": 17}
  ],
  "strands": [
    {
      "color": "#0066cc",
      "is_scaffold": true,
      "domains": [
        {"helix": 0, "forward": true, "start": 0, "end": 8},
        {"helix": 2, "forward": true, "start": 7, "end": 16}
      ]
    }
  ]
}

Note that because the above strand is a scaffold, and we want the strand to be forward on both helices, we must have both helices be even due to cadnano limitations on directions scaffold/staple strands can go on helices based on their parity.

However, the same error happens if the strand is a staple, and is reversed on both helices: