What does a BLACK triangle with exclamation point on text all messages page mean?

[Android Central Question]

I'm getting this symbol to the right of my texts with one particular person who has answered my texts in past. Have gone down and checked, and some texts did not send, so same thing as a grey or yellow triangle, or something different?

 

[ManiacJoe]

Which phone?
Which version of android?
Which carrier?
Which SMS app?