Again this relates to the noise coupling / sources.
If your output likes to hammer the local ground, adding
a capacitor to that ground (especially asymmetrically,
IN+ vs IN-) will couple ground impulses back to the
inputs, and now you must reject that by HF CMRR
(if balanced) or just eat it if unbalanced pin-C-Gnd.
Else you may get chatter despite an attempt at
hysteresis (a 5V output edge vs a few hundred mV
of hysteresis, comes down to the C*R high pass
corner as regards effectiveness of hysteresis).