# Errors in Process Models

**In short:** Many real process models contain control-flow errors, such as deadlocks and lack of synchronization. A model counts as correct only if it has neither. A typical cause is merging parallel branches with the wrong gateway type.

## Explanation

A process model can look fine and still have a structural flaw in how work flows through it. These flaws are control-flow errors. Research reports that many real process models suffer from them.

Two errors matter most:

- **Deadlock.** The process gets stuck. See *Deadlock in Process Models*.
- **Lack of synchronization.** BPMN defines it as a reachable state where some sequence flow carries more than one token. See *Lack of Synchronization*.

A model is sound, or correct, when it contains neither a deadlock nor a lack of synchronization. See *Soundness of Process Models* for more.

**Gateway mismatch is a typical cause.** A gateway is a BPMN element that splits or joins the flow (see *BPMN Gateway*). Suppose a parallel gateway splits the flow into several branches. An exclusive gateway then merges those branches. That combination breaks safeness, meaning multiple tokens can appear. *BPMN Gateway Mismatch Antipatterns* covers more of these patterns.

## Example

A loan team models two checks that run side by side: a credit check and a property appraisal. A parallel gateway starts both. An exclusive gateway then merges them before the final decision task. Both branches now reach the decision step separately, so a sequence flow can carry more than one token. Using a parallel gateway for the merge removes the flaw.

BPMN diagram: [A parallel split merged by an exclusive gateway: the final task can run once per branch.](https://arc.fuet.ch/diagrams/concepts/modeling-bpmn/errors-in-process-models/diagram-1.bpmn)

## Checks

- Each process model in the document contains neither a deadlock nor a lack of synchronization.
- No exclusive gateway merges the outgoing flows of a parallel gateway.

## Related

- [Deadlock in Process Models](https://arc.fuet.ch/concepts/modeling-bpmn/deadlock-in-process-models.md)
- [Lack of Synchronization](https://arc.fuet.ch/concepts/modeling-bpmn/lack-of-synchronization-in-process-models.md)
- [Soundness of Process Models](https://arc.fuet.ch/concepts/modeling-bpmn/soundness-of-process-models.md)
- [BPMN Gateway Mismatch Antipatterns](https://arc.fuet.ch/concepts/modeling-bpmn/antipatterns-for-process-flows.md)
- [BPMN Gateway](https://arc.fuet.ch/concepts/modeling-bpmn/bpmn-gateway-semantics.md)
- [BPMN 2.0 Model Quality](https://arc.fuet.ch/concepts/modeling-bpmn/bpmn-2-0-model-quality.md)

## Sources

- [BPMN 2.1 RTF — Open Issues - OMG Issue Tracker](https://issues.omg.org/issues/BPMN21-249) — issues.omg.org, accessed 2026-11-09
- [Instantaneous, Comprehensible, and Fixable Soundness Checking of Realistic BPMN Models](https://arxiv.org/html/2407.03965v1) — arxiv.org, accessed 2026-11-09