List of steps to reproduce (step by step, including full links if applicable):

• Install VisualEditor on a plain vanilla MediaWiki site
• Pick a page, and edit it with VisualEditor several times in a row, as rapidly as you can. Click Edit, make a small change, click Save, wait for the save to complete, and repeat. Usually 4-6 times is enough to trigger the following error.

What happens?:
This error dialog appears: Revision IDs returned by the server do not match (document: 4525, metadata: 4526) with buttons Cancel and Try again. I'll attach a screenshot so you can see the state VisualEditor is in.

The two integers vary, of course.

What should have happened instead?:
No error dialog

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:
MediaWiki 1.37.1
VisualEditor 0.1.2

Notes:

• I tested this on a plain vanilla wiki with no caching, and also a more optimized wiki with memcached. Same error.