Often you will see Git repositories with a top-level Dockerfile. Normally this is all nice and good. Still, when you do docker build -t image ., the whole current directory is the build context and is thus uploaded to the docker daemon. If you happen for instance to create test data (e.g. a few hundreds of megabytes of random data) in you current directory (possibly in a sub-directory), they will end up in the build context even if they are not referenced by the Dockerfile, wasting bandwidth and time.
Similar to the previous tip, as Docker will try to upload everything in the current directory, if it finds a file that it can't read, the upload will be abruplty interrupted. You'll see something like:
Uploading context
2014/09/19 15:17:24 Error: unexpected EOF