In addition to "I never met a man I didn't like", Will Rogers had another great little ditty I've always remembered. It went:

"It's not what you don't know that'll hurt you, it's what you do know that ain't so."

We all know or subscribe to many IT "truisms" that mostly have a strong basis in fact, in something in our professional careers, something we learned from others, lessons learned the hard way by ourselves, or by others who came before us.

Unfortuntely, as these truisms spread throughout the community, the details—why they came about and the caveats that affect when they apply—tend to not spread along with them.

We all have a tendency to look for, and latch on to, small "rules" or principles that we can use to avoid doing a complete exhaustive analysis for every decision. But even though they are correct much of the time, when we sometimes misapply them, we pay a penalty that could be avoided by understooding the details behind them.

For example, when user-defined functions were first introduced in SQL Server it became "common knowledge" within a year or so that they had extremely bad performance (because it required a re-compilation for each use) and should be avoided. This "trusim" still increases many database developers' aversion to using UDFs, even though Microsoft's introduction of InLine UDFs, which do not suffer from this issue at all, mitigates this issue substantially. In recent years I have run into numerous DBAs who still believe you should "never" use UDFs, because of this.

What other common not-so-"trusims" do you know, which many developers believe, that are not quite as universally true as is commonly understood, and which the developer community would benefit from being better educated about?

Please include why it was "true" to start off with, and under what circumstances it's not true. Limit responses to issues that are technical, where the "common" application of a "rule or principle" is in fact correct most of the time, or was correct back when it was first elucidated, but—in the edge cases, or because of not understanding the principle thoroughly, because technology has changed since it first spread, or applying the rule today without understanding the details behind the rule—can easily backfire or cause the opposite of the intended effect.