There's a (perhaps) apocryphal story about a king who found himself indebted to either a sage, or a peasant. (It's usually one of these two things.) For the rest of the story, I'll go with peasant, since that's how I first heard the story.
When prompted for her reward, the peasant woman merely said, "Take a chessboard, and on the first square, place a single grain of rice. On the second square, place double the rice of the previous square, and continue until all the board squares are done." But she was wise, and added, "Or, your kingdom."
The king thought this to be a simple reward! So he first gave one grain of rice, then two, then four, then eight, then 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384 ... well, you get the picture. So as you can imagine, the king eventually had to give his kingdom away to repay his debt.
The king and the peasant woman is a story of exponential growth, of course. But the consequences of even compound growth rates is something we often don't think of. So you'll see people saying they need storage that can start at 500TB and grow at 25% per year for 7 years without perhaps thinking about how much storage they'd need at the end of that time. And in reality, it's rare we can see past a few years growth without any real degree of accuracy.