Azure Data Factory Pipeline Debugging Fails with BadRequest

I recently had a new pipeline fail. It was actually a copy of an old pipeline where I had made some adjustments into as part of a database migration. When triggered during an execution run, it failed saying some expression could not be parsed. When I went into the pipeline and triggered a debug, it immediately failed with the following helpful error message:

Right. Not the same error from the monitoring pane. I went into a copy data activity, and found the following expressions:

The parsing error was from the expression @item.Query. I changed this to @item().Query, but the debug still failed with the Bad Request error. Strangely enough, validating the pipeline doesn’t result in an error. After searching the web, I found that the Bad Request error pops up when something isn’t configured correctly in the pipeline. A space in a name where it doesn’t belong, an unused variable etc. After some digging around, I found that there were some user properties in the Copy Data activity that referenced some activities that I had removed earlier. I fixed their expressions to reference existing items:

Personally I’ve never used the user properties before. They seem to be quite useful though, as explained in this blog post by Cathrine Wilhelmsen.

Anyway, once I fixed the properties the pipeline ran without an issue. I would have been nice though if the error was a bit more clear (for example, expression X contains a reference that cannot be resolved), or that at least the validation of the pipeline would fail.