So refreshing to see that others have this simple problem, too. Roombas really are like users. They will eventually find flaws and errors in your software, or, in this case, shoelaces and pieces of cloth lying on the ground.

Other issues with the roomba: - plant raisers with thin wooden feet. - wine rack with low base/feet - ikea chair with low U-shaped feet. Especially when the roomba tries to return to station

I also noticed that it has a hard time detecting dark/black surfaces. Probably it has to do with absorption of whatever kind of sensor wavelengths it uses.