These two tag synonyms are active at the moment:

• [thread] -> [threads]
• [threads] -> [multithreading]

The first synonym should be changed to [thread] -> [multithreading].

Probably completely automating such adjustments wouldn't be a good idea in case of bad synonyms (or cyclic relationships), but detecting and highlighting such cases for moderator intervention could be a good idea.

_{FYI: Currently a [thread] tag on a question is rewritten to [threads], not directly to [multithreading].}