Pluggable transports bundles warn of need to upgrade when no new version is yet available
Say there is a TBB and corresponding PT bundle both with version number X.Y.Z-alpha-1. https://check.torproject.org/RecommendedTBBVersions will say:
[ /* ... stable bundles ... */ "X.Y.Z-alpha-1-MacOS", "X.Y.Z-alpha-1-Windows", "X.Y.Z-alpha-1-Linux" ]
When the X.Y.Z-alpha-2 TBB is released, the file changes to
[ /* ... stable bundles ... */ "X.Y.Z-alpha-2-MacOS", "X.Y.Z-alpha-2-Windows", "X.Y.Z-alpha-2-Linux" ]
If we haven't built new corresponding PT bundles yet, those will still have the old version number X.Y.Z-alpha-1, and users will get a blinking Tor button telling them to upgrade. However no upgrade exists for them yet.