When you are writing a template engine for a new site, and want to test a bit of logic: the following will fail:
{%if 1>2%}
Logic works
{%/if%}
Why? Well, 1 is not greater than 2! Took me nearly 15 minutes to try and figure out why “Logic works” wasn’t showing – the logic was working, but my brain obversely wasn’t at that point!