Hello,

I often have this problem, but I have a stop-gap solution.

This time I'm trying to make a document with different styles, but the ToC was messed up.
The styles I'm using are the basic styles given by Word 2013.
I'm using the default Normal, No Spacing, Header 1, 2 and 3.

The ToC had a line that didn't belong there.
Apparently that line was part of a table I have and the style applied was Header 2.

I have tried many times to change the style by selecting the text and clicking on No Spacing, but it just changed to Header 1 so it got even worse.
The font size, type and colour change, but the tag applied is still the same, so in a ToC it still shows up.

The stop-gap solution is simply to remove the corrupted part and retype, which I have applied already.

I have had this problem a couple of times before though and sometimes clicking on the forced style and then clicking on the desired style worked
Rebooting Word doesn't help. I haven't rebooted my laptop, next time I encounter this problem I'll try that too.

I am thoroughly annoyed by this, so I'm asking if others have encountered this or if it's just something quirky that my PC or Word throws at me to ruin my day.

Turn on display of non-printing formatting characters so you can select a whole paragraph.
Showing non-printing formatting marks in Microsoft Word
Try Normal or Body Text and select the paragraph, not just text. If you apply a linked style to selected text it does not change the outline level of the paragraph.