Studio version: 24.10.9 (C# project)
If there is an orphaned opening brace { in any activity, the whole workflow up to that point breaks with CS0106 errors.
Ironically, the activity with the orphaned brace does not have a validation error.
It’s not a bug per se, as it’s clearly a “user error”, but more of an unintended behavior when the user does make this error.
Repro:
- Create a new workflow.
- Put any activities in it.
- Add a LogMessage activity (or assign or anything else) with this as the variable value:
{"Break here" - Marvel in the sea of validation errors

As you can see in the picture below, it is not easy to find the error causing char if you don’t know what you’re looking for (diffs for the win).
EDIT:
For lost souls trying to search for this in the future by error message:
CS0106: The modifier 'public' is not valid for this item

