Polygonaceae Fallopia sachalinensis
Originally uploaded by Leonieke.
Found this on Flickr while searching for photos for the Life on the Japanese Knotweed project. The title indicates that this is Giant knotweed (Fallopia sachalinensis, Polygonum sachalinese, etc.), but the leaves tell a different story. Giant knotweed has very large, obviously "cordate" (heart-shaped) leaves with slightly wavy margins, while Japanese knotweed (F. japonica) and the hybrid Bohemian knotweed (F. x bohemica) have leaves that are more squared-off at the base, like you see in this photo. All three are invasive outside of their native Asia.
The interesting thing here is that it's not a simple photographer's error - if you look at the largest size of the photo you can see that the botanical tag actually says Fallopia sachalinensis. This photo was taken at the botanical gardens in Leiden, The Netherlands, one of the oldest botanical gardens in the world...they'll be getting an email from me soon - I am very curious.
Bonus points to Leonieke for putting this and other photos up under a CC license.