QGIS » QGIS Application

Since QGIS 2.6, the content of a map item will incrementally shifts downwards as the zoom level increases when starting at 800%. It effectively prevents using an extreme zoom value to fine-tune the location of a composer element (say an arrow item) and the content of a map item.

1. Open an existing QGIS project
2. Create a composer, or open an existing composer which has a map item
3. Make sure the map item you'll use is set to "cache"
4. Set the composer zoom level to 800%, pan the composer sheet so the map item's content is visible, and hit the map item's "Update preview" button
5. Set the composer zoom level to 825%, and hit the map item's "Update preview" button again
6. You'll notice the map item's content begins to shift downwards
7. Repeat the operation with zoom levels 850%, 875%, 900%, etc. to see the map item's content slowly sicking downwards :)

I'm setting this as a blocker, Nyall, feel free to downgrade if you think it shouldn't be set to such a high priority.