I have experienced this situation several times in projects, and I believe that often it is due to a lack of understanding of these seemingly simple parts of the project that may take much more time than is budgeted for. From my experience, this often happens with processes related to our Tier 2 suppliers. For example, our suppliers may outsource certain processing steps, such as metal coating, or may purchase parts for our products from a tier 2 supplier. Sometimes the project manager “thinks” they have a good estimate of the time it will take for these outsourced processes or procured components based on past experience and never follow up with our supplier to double check. Later down the road, when it comes time to execute the processing step at the outsourced vendor or procure a part from an outsourced vendor, they find out that the lead time was 4X what they were expecting. Immediately this processing step then becomes the critical path. The lesson to be learned from this is never to assume a lead time for any process, no matter how simple it seems.