One of the recurring topics in the forums is rounding of floating point calculations. I just recently saw a post in slashdot referencing this article:

http://www.techradar.com/news/computing/why-computers-suck-at-maths-644771

It points out things like: According to Google 599,999,999,999,999 – 599,999,999,999,998 = 1

And tells this story:

The case of a Patriot defense system failing to take down a Scud missile attack: 'The calculation of where to look for confirmation of an incoming missile requires knowledge of the system time, which is stored as the number of 0.1-second ticks since the system was started up. Unfortunately, 0.1 seconds cannot be expressed accurately as a binary number, ...

[snip]

...and 28 people paid with their lives.'"

Good teaching examples, and much more accessible than The Standard Reference (What Every Computer Scientist Should Know about Floating Point Arithmetic)