Just hit this:

The mod Belt Overflow (1.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-overflow::on_tick (ID 0)
Entity is not transport-belt-connectable.
stack traceback:
[C]: in function 'get_transport_line'
belt-overflow/control.lua:235: in function <belt-overflow/control.lua:207>

More detail: Seems to happen when I try to place underground belts. If you mix ghosts and regulars, you get the crash pretty reliably. So, place 2 ghosts that match, 1 down, 1 up. Then try to place an actual underground for either end, and you get the crash.

This should be fixed, along with some other ghost-related bugs, in the latest version.