Capacitively, pin-to-pin, but principally via the breadboard I'd
assume.
Consider: that's a 50mV step into roughly 20pF. A stray of 20pF x
50mV/(3.3V logic swing) = roughly 0.3pF of cross-coupling needed to
produce that. You mentioned earlier that your solderless breadboard
supplies 2pF.
I s'pose 'tisn't usually a problem unless the GPIO transition comes
when the oscillator input's close to its threshold, and is of the
opposite polarity (same polarity just triggers the oscillator's output
edge early, while opposite polarity produce a glitch).
Absolutely yes! For example, a 10 megohm oscillator input Z (not
unusual) x 20pF yields a 200uS time constant ! (But *your* waveform
suggests about 4uS t/c, best I can tell, suggesting a Z near 200K.
Your earlier reports also indicate a low-ish input Z.)
