1. Inaccurate Estimations: This happens when the project's budget is not accurately estimated at the start. This could be due to a lack of understanding of the project's requirements, leading to underestimation or overestimation of the resources needed.

2. Scope Creep: This occurs when the project's scope increases beyond its original objectives without adjustments to time, cost, and resources. This often leads to budget overruns as more resources are required to fulfill the expanding scope.

3. Poor Planning: Insufficient planning at the beginning of the project can lead to unexpected costs down the line. This can involve not considering all aspects of the project, such as labor costs, material costs, and overheads. 

4. Change in Requirements: If project requirements change midway, it can result in increased costs. The additional costs may not have been included in the initial budget, leading to budget control issues.

5. Lack of Risk Management: Failure to anticipate and plan for potential risks can lead to unexpected expenses. These could range from sudden price changes in resources, unexpected delays, to natural disasters.

6. Ineffective Monitoring: Without proper monitoring and control mechanisms, it's easy for a project to exceed its budget. This could be due to a lack of regular budget reviews, or failing to take corrective action when necessary.