Reply to does not always include the entire previous message
Reply to does not always include the entire previous message. Some emails I have to reply to from Gmail because either a) the reply email has no content at all (just the sender and a blue 'bar' that marks the beginning of the email I am replying to) or b) the last message is not included in the reply, so it looks like I am replying to myself.
I can't see a consistent pattern as to what causes this. If I see anything I will let you know.

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David Shulman commented
I am having the same issue
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Martin Scholz commented
I have that too every once in a while. My best guess is that it relays to the feature that - if you mark some part of an email before hitting "reply", it only includes the marked/highlighted part in the answer. So my workaround: if you mark all content of the original email (e.g. by "Cmd+A") before hitting "reply", the marked text is included.
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Matthew Cawley commented
If you select any text in the email and the press reply then only the selected text is shown in the reply. It could be possible that you have double clicked on empy space in the email without realising some time (which may select an empty line).
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Anonymous commented
Are you highlighting some content before hitting the reply button? I usually notice when I do that only the highlighted text is quoted in the reply to message
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Sören commented
I have the same problem in our company. when the eco reply's or forwards a email, sometimes the original email text is missing.
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Joseph commented
I'm also seeing this. Sometimes it's totally gone...and just shows date, name and "wrote:" and then nothing.
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Nik commented
This is caused by the "select text to be quoted in reply" feature. Sometimes there appears to be a selection of nothing, therefore the reply does not include the previous message. I then have to select-all, then replying.