If there is an issue with a smart field within a text box, that smart field can break the entire form, email, or task where the smart field is placed. If this happens, you will need to head to the template locate the broken variable. A variable can break if there is an extra or missing bracket on either end, the text within the variable was edited, or the format of the variable is not consistent throughout the variable. 

The client name smart field should look like {{client.name}} but some broken smart fields can look like....

Example of invalid brackets:
{{ $1000 }}
{client.name}}
{{ Jeff }}
{{{ client.name }}}
{{client.name}} (part is formatted differently)

What it should look like:
{{client.name}}

Head to the template or form that is applied to the client and click edit. Click on the text box that is having trouble. One quick way to check would be to open the finder in your browser (CTRL+F or COMMAND+F) and search for brackets {{ or }} if you see an odd number of brackets the issue could be an extra or missing bracket. If not you can press Next in the finder and look at each Smart Field for Formatting issues.

Again, be sure you are not making up any smart fields that are not in your SMART FIELD dropdown. 

On a proposal a broken variable will show up as an empty box. You will need to find that box to adjust the broken package: