[noisymime]

You cannot just disconnect the wire and have the input floating. If an input has nothing connected to it but is set for interrupt monitoring, it's just going to fire randomly based on electrical noise. This doesn't indicate a problem, it's just how electronics work.

Super basic electronics lesson: if you have an input on being monitored, it must have something connected to it. Connect the pin to earth and you'll see that everything is fine.

[hello]

Didn't processors change since the '40's,
pull-up, push-up, ice cream, you scream

but I don't know exactly why one pin should be any different from another,
but there behavior is.
.....
.....
my preschooler says the board looks really ugly, she could do better, see attached.

[theonewithin]

Ok...


This guy must be just trolling right???

[Alfagta]

No real input from this user...
Ignore until there is coming something real...

[StacyPoli]

Hello...I think there are some pins on a Mega that do not have pinChange interrupts. Maybe you have accidentally stumbled on them? Have a look at the Atmega 2560 datasheet.

https://www.7pcb.com/