In 1999, when I first bought an HP49G, whose major selling point was a CAS, I thought I'd try summing the harmonic series $\sum_{n=1}^\infty \frac{1}{n}$. I was a bit surprised to see the result 1151.8697216.
It turned out that it knew how to numerically compute the discrete antiderivative $\Psi(m) := \sum_{n=1}^m \frac{1}{n} \approx \ln m + \gamma$, and in the particular mode that it happened to be in, it would replace $\infty$ with the largest floating-point number it could represent, which was just under $10^{500}$. Indeed, $\Psi(10^{500}) \approx 500\ln 10 + \gamma \approx 1151.8697216$.
The story has a happy ending: after changing some flags, it returned $+\infty$.
