Paying attention to negatives.

Last night I emailed Hiro from my mobile in Japanese intending to say:

I am going out but won't be late coming home.

He was taken aback to get the message:

I'm going out, will be late and won't come home.

Ooops
Memo to self - take care with forming negatives...

Almost as bad as the day years ago when I intended to tell a fellow teacher:

No no, no need to share your umbrella with me, when two people use the same umbrella they invariably both get wet.  (nureru is the word to get wet).

but actually said:

No no, no need to share your umbrella with me, when two people use the same umbrella they invariably both get naked. (nugeru is the word to take off your clothes.... ).

She looked at me oddly, but said nothing ... I realised several hours later.....