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...