Some versions Microsoft Outlook incorrectly quote bottom-posted e-mails, causing your reply to be lost in the line noise -- it's greyed out, like the quoted text above it. This bug caused me no end of problems until I found out about it, in 2009.

I had repeated customer complaints about "Blank e-mails" or missed content in my inline replies.

Until the very last Outlook user disappears, I can't risk bottom-posting in business correspondence. You can thank Microsoft for destroying the legibility of e-mail threads.