I am doing some research into common errors and poor assumptions made by junior (and perhaps senior) software engineers.

What was your longest-held poor assumption that was eventually corrected?

For example: I at one point failed to understand that the size of an integer was not a standard (depends on the language and target). A bit embarrassing to state, but there it is.

Be frank: what hard-held 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, anything under the computer science domain.