I am doing some research into common errors and poor assumptions made by junior (and perhaps senior) software engineers.
What was your longest-held assumption that was eventually corrected?
For example, I misunderstood that the size of an integer is not a standard and instead depends on the language and target. A bit embarrassing to state, but there it is.
Be frank; what firm belief did you have, and roughly how long did you maintain the assumption? It can be about an algorithm, a language, a programming concept, testing, or anything else about programming, programming languages, or computer science.