When I used to pre-heat my oven to 350F, I couldn't help but wonder if the temperature was as promised when the oven beeped. To quell my fears, I bought a thermometer and placed it on a rack inside my oven oven (on the outermost bar, near the door). I felt pretty smart when I found out that it took 10-15 minutes longer after the oven beeped to hit the target temperature. That's why I was having so many problems!!
We since moved, and I brought my thermometer. I put it in the oven here, and as I learned its quirks, I realized it similarly took longer to heat up than promised by the oven's thermostat. But I was having other trouble with the oven, that involved heat, smoke, and char everywhere (food and oven walls). On a whim, I put the thermometer further into the oven (now hanging on a rack in the very center of the oven), and I find out that the oven is hitting then exceeding the target temperature by 25, 50 or more degrees!! My 400 oven hit 500 (and beyond?)!
Now, I'm not sure where I'm going from here, but knowing is the first step. I've been much more cautious with high temperature cooking, which can lead to burning, smoking, and splats (aka dirty oven). I've considered looking into getting it repaired, but we're thinking of moving soon, so it might not be worth the expense.
Whatever I do now, the take away lesson: Never take the temperature of your oven for granted. Whether too high or too low, it can affect your cooking, and in turn, your confidence in cooking.