A tendency to underestimate the time, resources, or effort required to complete a task, despite having knowledge of past experiences where similar tasks took longer than anticipated

Examples:

“Would you care to guess how many students finished on or before their estimated 50%, 75%, and 99% probability levels? 13% of subjects finished their project by the time they had assigned a 50% probability level; 19% finished by the time assigned a 75% probability level; and only 45% (less than half!) finished by the time of their 99% probability level. … The Denver International Airport opened 16 months late, at a cost overrun of $2 billion.1 The Eurofighter Typhoon, a joint defense project of several European countries, was delivered 54 months late at a cost of $19 billion instead of $7 billion. The Sydney Opera House may be the most legendary construction overrun of all time, originally estimated to be completed in 1963 for $7 million, and finally completed in 1973 for $102 million. “ – Planning Fallacy Sequences

Related and more general: Planning overconfidence