Misconception that something will be simpler than it really will be. 
Leads to missing deadlines, demoralization, depression, and DeathMarch projects.

Some causes. (there are many more)

* Human nature. I.e. wanting things to be simple.

* Picturing progress within a static environment. I.e., the work moving forward while the context stands still. Context in this case means, the requirements, the marketplace, the changes incurred to the requirements by the very process of moving forward.

* Ignoring the prep work required before you can actually launch into something. "this will be so easy once I connect all the dots", failing to realize the labor involved in, or failing to quantify entirely, the prep itself.

* Ignoring troubleshooting and debugging altogether.

* Grouping things in false batches, i.e. generalizing, thereby ignoring case-by-case work you will end up HAVING to do (i.e. ignoring granularity or conflicting needs), EightyTwentyRule

* Similar to the previous point, focusing on the broad strokes and ignoring the details.

* Misaligned expectations: Your vision was different than the owner, manager, or customer's vision.