Overheating. No output. Smoke. Off and on buzzing. All signs that something is most certainly wrong.
In this case, I was working on building my synth, and I had the oscillator working. I had a 555 diagram all laid out to make a square wave, and it worked! I had to use other parts than what was described (replace a 1M pot with a 50k and a .02uF cap with a .22nF cap). But, using the 50k pot I noticed I couldn't get very low frequencies (I don't care if it can oscillate at 10kHz, I want like 1000 and less), so I thought that a higher rated pot (250k, my highest) would get lower frequencies.
So, I wired the new pot and breadboarded it, but I accidentally messed up the power situation and swapped the negative and positive (oops...). At first, it was sputtering - I thought it was a loose connection. As I let it run while checking connections, I noticed the smell of hot wires (similar to soldering). I wasn't sure what I was smelling. Maybe I had been working with wire and was imagining the smell. But then it happened.
A small, nearly invisible puff of smoke arose from around pin 1 of the 555.
Hoping it was actually a wire and not the timer, I simply released the power and waited a bit. I then put my finger on the 555, and almost immediately burned myself. I swapped the power supply around, thinking that I may have messed that up. It was able to burn me in less than a second. I took everything apart and noticed something: the breadboard under where the 555 was was partially melted! That's hot.
Lesson learned: power is a dangerous thing, even at only 9v, and making sure it's going the right way makes all the difference.
I'll get another 555 later and try it again. At least I know the square wave schematic works!