We introduce LabelAId, an ML-based inference system to provide just-in-time feedback during crowdsourced labeling to improve data quality and user expertise. LabelAId consists of (1) a novel ML-based pipeline for detecting labeling mistakes, which is efficiently trained to infer label correctness based on user behavior and domain knowledge; (2) a real-time ML model and UI that tracks worker behavior and intervenes when an inferred mistake is occurring.