Factorio Forums

See also test map.

step 1: make a crossing with one rail from bottom left to top right (so diagonal) and one from bottom to top (vertical)
step 2: place a signal on the diagonal rail, left from but as close to the vertical rail as possible
step 3: place a corner on the diagonal rail, as close to the signal as possible, from bottom left to right (going from diagonal to horizontal). The signal starts flashing, because it is too close to the corner
step 4: remove the corner. Signal keeps flashing: bug!

Seems similar to the issue I've ran into. See picture.
It seems like adding the curved rail makes the whole contraption one big block and it stays that way even after removing it.
To fix the issue you need to replace the chain signal. Also it's not possible to place a signal in the offending possition if the rail is already built so the signal should probably block construction of the rail, no?

Fixed for 0.15.