Terrorism - admitting that it works is too hard to take...

Progress is often stifled by pretending to live in a world that one would prefer to live in, rather than the world one does live in. For example, the UK is saddled with a political and administrative entourage suited particularly well to a powerful, wealthy nation running a large empire, but singularly ill-equipped to manage a post-industrial society sinking under a load of debt.
Another example - hallucinogenic drugs make people feel good and don't do any lasting harm to most of the people who use them. Include alcohol and tobacco in this definition, as they should be included, and it's patently true. Now, this doesn't excuse naive scientists who tangle with governments, but it does explain why any policy which refuses to acknowledge this fact is bound to fail.
And here's a big one to follow up with - violent terrorism gets results.