Microsoft’s Bing aerial imagery (that’s recently been donated to OSM for use in tracing) is offset by a few meters in some places.
For example, earlier this week I traced buildings with imagery from USDA’s NAIP program. Besides a history of being well-rectified, a GPS track in the parking lot confirms that I was spot on. However, on Bing with a low zoom, they’re misaligned:
Zoom in a bit, and they magically align again:
Apparently, different, well-rectified imagery is used at higher zooms. If I’ve found one problem, it then follows that it exists elsewhere. Be careful when tracing!