vt. To absentmindedly forget to do something.
Spacing something out is like blowing it off, but is less intentional; see blow off
I meant to buy batteries when I went to the store, but I spaced it out.
*** Dear UD staff,
You need to fix your software.
Your user interface is telling me (falsely)
that this example doesn't contain "space out."
If you need a programmer who's up to the task,
drop me an e-mail and I'll send my resume.