The above image holds the crash log, as well as the situation which caused it. I placed the Heat Pipe under the red circle, then the crash happened. I had been placing and replacing Underground Heat Pipes for some time, trying to clean up a design. Some may have been forcibly replaced via in-game copy/paste. There used to be an Underground Heat Pipe below, where the Heat Pipe corner intersection is in the screenshot, but I removed that shortly before.

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Actually, it seems like my game crashed every time I try and place an Underground Heat Pipe segment in that specific slot. This suggests I've somehow left a broken invisible piece of a previous Underground Heat Pipe somewhere that this is trying to connect to. It doesn't seem to matter which direction I face it, either. Same crash every time.

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Looking at control.lua line 103, it doesn't really make sense what could be crashing. It says invalid lua entity, but you're already checking old_directional_heatpipe and new_directional_heatpipe was just created. So unless LuaSurface.create_entity is somehow giving an invalid entity, the only thing I can figure is old_directional_heatpipe is somehow both not null and also not valid. It's just THAT one tile. Everything else works fine, so I suspect you're getting garbage data from surfacedata.directional_heat_pipes[key], but I'm genuinely guessing at this point.