5 DataWeave Array Traps That Silently Destroy Your Data

Last October I reviewed a Mule integration that had been running for 3 months. The flow pulled employee records from Workday, filtered active ones, and pushed them to an HR portal. Everything looked clean. No errors in CloudHub. No alerts. The downstream portal had 2,400 employees. Workday had 4,200. Forty percent of the records had been silently dropped. The filter was the problem, and the fix was one character. I found 4 more traps like this in the same project. All silent. All invisible until someone compared the numbers manually. Here are all 5, with the exact code that breaks and the exact code that fixes it. TL;DR filter silently drops records when you compare String to Number (no error, no warning) reduce with wrong accumulator shape gives you wrong totals for weeks groupBy returns an Object, not an Array — chaining map after it throws a cryptic error distinctBy keeps the first match and silently drops duplicates from other systems zip truncates to the shorter array — extra reco