Everyone who has worked on a project has a story where a critical path was somehow delayed for varying reasons.

As someone working in the hydrophilic coatings industry for a while, I had a lot of customers think of my product as an afterthought when it was in fact a very specific and detailed portion of the medical device. When they found out that applying my product to their device would require extensive process design and V&V, they became quite surprised. Suddenly, my insignificant coating was on their critical path. In severe cases, this even sunk their projects.

Can you think of a case where something that was believed to be an extraneous task suddenly wound up on the critical path for the project?