I've encountered an annoying bug where the red lines that preview a marked area are persisting after conversion to marked area and subsequent removal of markers & machine. (Screenshot attached)
I placed landmarks to mark out an area, connected them, and then placed a filler next to one of the markers to begin clearing out an area. I noticed that, while the yellow/black striped lines appeared, the red preview lines did not disappear. I didn't pay it much mind at the time, but when I later removed the machine & all leftover markers, the red lines continued to persist, even though the striped ones were removed as expected. Reloading the world did nothing to solve the issue.

Screenshot:

BuildCraft version: 7.99.18
Forge version: 14.23.5.2768
Link to crash report or log: n/a
Singleplayer or multiplayer: Singleplayer
Steps to reproduce: Unknown

In theory removing the markers should destroy the outline although they are stored separately so it might be an issue where removing one doesn't remove the other. Does this persist through quit+relaunch? Also does that happen every time that use remove any markers, or did this only happen once?

Having been forced to forcequit Minecraft due to a freeze-up (unrelated to BC), I can confirm that a complete relaunch does indeed fix the problem. As for repeatability, this has only happened once.
If it's useful information, I should say that the bottom rectangle was connected before I placed the other markers.