Read back
Not /forgetting too read back a freq is certainly not uncommon especially if you, in anticipation, already have the freq set up in standby on the radio.
ATC clears you from his freq, gives new one - you glance at the radio to confirm it is as you entered it.
Say goodbye to ATC, forget to read back the freq.
Switch freqs on the radio.
Not reading back the freq is not going to cause any reaction on the part of ATC.
Certainly no: "My Gawd he didn't read back the freq! We had better scramble the fighters!"
If by analysis you mean endless posts of speculation, let's not do that.