Loopy, forgetful, absentminded, scatterbrained... the last one is my favorite. For example, "My roommate was so scatterbrained yesterday, she put her shirt on backwards and forgot all her homework on the kitchen table." Absentminded is someone who's not really paying attention, scatterbrained is someone a little more chaotic.

Without a context, "forgetful" the other words that Mindy suggested could all be correct in the appropriate context, but without that, err on the side of tactful. (My personal favorite is "dizzy", but that's another story.)

Forgetful

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"Overlook or Amnesia"