Example
*"First, no evidence exists that water exists on Mars. Secondly, ..."
What's wrong?
You've enumerated a list of items using markers such as "First," "Second," and so on, but they aren't consistent. Use first, second, third or firstly, secondly, thirdly; don't mix them.
How to fix this problem
Use consistent enumerators If two of the enumerators are separated by a lengthy passage, consider reminding the reader what's being enumerated, as in the following example: Today's voting technologies are controversial for three reasons. First, ... (yadda yadda). The second reason voting technologies are controversial is...