I've started to notice a strong yellowing of the indoor pictures with my Canon Rebel T1i. If I use the flash, this doesn't happen, but I don't want to have to use the flash on every picture as this really annoys people. Did I do something wrong to the camera?
It's the lights indoor, they're usually "warmer" and that lends a color cast to images that the camera can normally compensate for. What you want to look for is the "white balance" settings for your camera. I suspect that yours has been set on "Flash" rather than "Auto".
The manual, if you don't have it, is available online. Page 96 should have the info.
You can, once you know this, set your camera according to the lighting any time you want. For the most part, auto is usually okay.