We had a problem where a job would not pick up changes from git. It turned out that:

hudson.plugins.git.GitAPI#revParse(String revName)

just takes the first line of output from the rev-parse command into account.

Our problem was that someone had created a tag with the same name as the master branch. So we had 'origin/master' as the name of a tag and as the remote branch. This causes rev-parse to output:

$ git rev-parse origin/master
warning: refname 'origin/master' is ambiguous.
1a7b6f67b971f01f78c685f3d0917ea623fb5122

which stopped the plugin from figuring out what new revision should be. The problem was that this piece of information was not output in the build logs so was a bit tricky to track down.

I'm guessing that a simple check for this output and an appropriate exception could be added to GitAPI to aid in tracking this kind of problem down.