the economic principle of the tragedy of the commons. Supporters say that despite its initial problems, Vélib has become a model of innovation in urban public services. Maintenance Issues have surfaced on how to safeguard riders from hiring a returned and damaged Vélib cycle or a cycle requiring immediate maintenance. To indicate when there is a problem with the bike, it has become common for returning users to rotate the seat through 180 degrees to point backward. While this practice assists staff in determining which bikes require immediate attention, it depends for its success on substantial voluntary user compliance. Timing differences of up to 30 minutes exist between clocks of different rental stations due to poor synchronisation, and may give rise to overbilling or underbilling. The mayor of Paris has given assurances that overcharges will be reimbursed. Rental stations use the Microsoft Windows operating system and have been known to crash, giving the infamous blue screen of death. Demand There are frequent disparities between the availability of rental bicycles and the number of rental slots. Each rental station indicates the number of bicycles available at the nearest stations. Demand can be high during the working week and during transport strikes. Although rental bicycles are not dedicated, it has been known for people to chain a bicycle to its station so that it will remain available to them for a subsequent trip. Vélib' has declared this to be uncivic behaviour, and Vélib' employees are authorised to cut the locks in these cases. Stations at greater elevations generally experience greater demand. In addition, there is a net inflow of bicycles from the outskirts to the city centre earlier in the day, and a net flow outwards in the evening. Thus, depending on the time of day, Vélib needs to manage the demand at its outermost and centremost stations. Consequently, bicycles may be completely unavailable in some locations, whilst parking problems exist for others. Vélib' faces logistical issues, and must relocate bikes during the course of each day. There are proposals to make adjustments to the system's pricing mechanism. The "bonus V'+" bonus system was put in place on 14 June 2008 in an effort to adjust the demand level. Fifteen minutes of free cycling time is credited to users who rent bicycles from stations without the logo (mostly situated at the edge of the city and more than 60 m above sea