i've always been a baker

Not professionally of course, but I've spent a lot of time around ovens.  Now that I have a place of my own where the kitchen is organized to my liking and clean all the time, I find myself cooking more often than not.  Baking has taken the back burner.  This particular post comes because I am currently baking something for a friend's birthday (yes at 9 am,  I just got off of night shift). 
The funny thing about my new old oven in my condo is it doesn't have a way of letting me know when it is preheated.  I have no idea what the inside temperature is.  Or at least I didn't used to.  About two months ago I went out and bought an oven thermometer.  I've been quite surprised to learn that ovens aren't to be trusted just because you set them to a certain temp. 
Take it from someone who knows cooking:  The Chefs at Cooking Light
"Ideally, every oven set to 350° would heat to 350°. But many ovens don't, including expensive ones, and some change their behavior as they age. Always use an oven thermometer."  It's one of the top 25 most common cooking mistakes!!
Now I have to use this nifty gadget because my archaic oven would have it no other way, but I dare you to try a thermometer in your oven just to see if that preheated light really turns on at the correct time.  They make specific ones just for ovens that either hang or sit on the racks.  You can find them anywhere from the grocery store to crate and barrel.