Making this setup and then placing rail signals in the white spots will allow signals to be placed. When the game saves, this causes the game to crash. Both auto-save and manual saving will crash the game. Tested on different systems to be sure. Screenshot of setup and log attatched.
Normal setup, allows for wrong signal placement.
Signals placed in the wrong spot. This causes crash when saving.
It appears that the invalid signals be placed in 4 positions, dependent on the curved rail direction.
Valid signals can be placed around it, as expected, but placing one on the same tile appears not to split the train block, and only one of the two signals interacts properly with trains.
The game autosaving appears to crash the game, and result in a partial, corrupted autosave. However contrary to duplex, a manual save appears to complete fine.
Save-file from setup. Fresh world, no mods, enabled /editor, placed a chest with 100 rail, 50 normal and 50 chain signals. Exited editor mode and made the setup as shown in these screenshots.