How do I avoid erroneous save conflict on a work item?

I'm trying to change a Task work item from "In Progress" to "Complete". It never works. No one else is modifying this work item. Most of the other work items seem to work OK.

In the web, I press the refresh icon, then change the state in the drop down box, then press Save. It says:

Save conflict. Failed saving work item 9676 because it did not have the most recent state. Refresh, make additional changes, then save again.

In Eclipse, I press the refresh icon, then change the state in the drop down box, then press Save. It says:

Data changed remotely

The data you were about to save did already change remotely and your changes could not be merged automatically.
CRJAZ0223I The following item has been changed by another operation:
Input: com.ibm.team.workitem.common.internal.model.impl.WorkItemImpl@e5807012 (stateId: [UUID _N91x4XzBEeWMpbya1DuQ9Q], itemId: [UUID _UmJgRWt2EeWQxIpNOszLUw], origin: <unset>, immutable: true) (contextId: [UUID _swapoFq7EeSFcN_Ld41kWg], modified: 2015-10-27 11:41:41.246, workingCopy: <unset>) (mergePredecessor: null, workingCopyPredecessor: <unset>, workingCopyMergePredecessor: <unset>, predecessor: [UUID _pRNnsHbHEeWMpbya1DuQ9Q]) (id: 9676, internalState: 3, internalResolution: , resolutionDate: 2015-10-27 11:41:41.23, summary: Fix something something something, internalSeverity: severity.literal.l1, creationDate: 2015-10-05 11:33:11.228, internalPriority: priority.literal.l01, dueDate: null, description: , workflowSurrogate: , internalTags: , workItemType: task, duration: 28800000, timeSpent: -1, internalSequenceValue: null, correctedEstimate: -1, startDate: null, archived: false)
Current: com.ibm.team.workitem.common.internal.model.impl.WorkItemImpl@3c3566a2 (stateId: [UUID _yDQ20W3tEeWHsIhR5fxsmg], itemId: [UUID _UmJgRWt2EeWQxIpNOszLUw], origin: <unset>, immutable: true) (contextId: [UUID _swapoFq7EeSFcN_Ld41kWg], modified: 2015-10-08 14:52:53.821, workingCopy: <unset>) (mergePredecessor: null, workingCopyPredecessor: <unset>, workingCopyMergePredecessor: <unset>, predecessor: [UUID _P3jpkGvLEeWQxIpNOszLUw]) (id: 9676, internalState: 2, internalResolution: , resolutionDate: null, summary: Fix something something something, internalSeverity: severity.literal.l1, creationDate: 2015-10-05 11:33:11.228, internalPriority: priority.literal.l01, dueDate: null, description: , workflowSurrogate: , internalTags: , workItemType: task, duration: 28800000, timeSpent: -1, internalSequenceValue: null, correctedEstimate: -1, startDate: null, archived: false)