When you open the save, you see repair bots trying to repair a damaged wall, but running out of power before they reach it.
I had one huge bot network that I started splitting into smaller parts. I think what happened is the bots had a repair order when I moved the roboport and broke the link with the area they wanted to repair. Now they keep trying to reach the walls that are damaged, run out of energy, and instead of taking the nearest roboport (because that is not in their botnet), they go all the way back to their original roboport (at very slow speed), then charge up and try again (and fail again). When I delete the bots, the damaged walls are not reassigned to another bot, and stay damaged.
Tested it with no mods, Win 10 x64 zip.
[0.12.7][posila]Splitting bot network confuses repair bots
Re: [0.12.7][posila]Splitting bot network confuses repair bots
Thanks for the report. In 0.12.9 construction robots should be transferred to appropriate network when a logistic network splits.