When I set up a simple low-speed combinatorial pulse detecting circuit using HC-logic on a breadboard I noticed that especially the falling edges were horrible and caused glitches. The edges had some sort of oscillation on them (unlike ringing) and I supposed they were due to bad contacts of the breadboard but whatever I tried I couldn't get rid of them until I finally soldered it all together.
Now my questions: _how_ do bad contacts do this? And why only the falling edges?
As Jack hinted typical breadboards such as the white "plug-and-go" boards or whatever they are called aren't good enough anymore for modern logic. Take a look at VCC, maybe you'll see ringing there as well.
Anyway, a photo and a schematic would help. I really don't think it's bad contacts. Then you should be seeing changes when you tug on the wires.
Original breadboard circuit without C2. The output pulses had weird oscillating rising edges. As Joerg proposed, closer inspection of the power lines reveiled some 100MHz or so ringing on the 5V and GND. After the addition of C2 the problem went away, the ringing became one period of "slow" 20MHz-ish bouncing. I had placed decoupling caps (100n & 10u) as close to the 74AC14 as possible but they didn't seem to have any influence.
There were also clamping diodes just after C1 but they didn't influence the behaviour so I left them out for clarity. The purpose of R1 is protection for the schmitt-trigger and its value doesn't seem to have any influence.
BTW, I made an error, it is on the rising edges only, not on the falling edges.