Is there ever a time when the reading from a sensor isn't a number?

I'm writing some code on the web server to handle readings from different sensors. Just out of curiosity, are there ever any situations were a sensor reading wouldn't be some kind of number? I can't really think of any.

Depends on the sensor. The value from an analog input is an integer. The value from a digital input is 0 or 1. The value from a sensor connected via Serial, SPI, or Wire could easily be text.