I have a profile on the launcher for snapshots specifically, and I noticed that the pre-release was out. However, after downloading the pre-release, I noticed that my main profile (no snapshots) also ran the pre-release. I then noticed that 1.7.3 had downloaded as a release and not a snapshot!
Steps to Reproduce:
1) Create a profile without snapshots, and select "use latest version". Notice the latest version is release 1.7.5
2) Create a new profile and enable snapshots. The latest version is 1.7.6pre1
3) Run the snapshot profile
4) In the non-snapshot profile, the latest version is now 1.7.6pre1
Note that this only happens with pre-releases, and also occurred with 1.6.3.