What is your longest-held programming assumption that turned out to be incorrect?
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.
What is your longest-held programming assumption that turned out to be incorrect?
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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.