A search that begins with the string
votes will cause the search parameters to replace that string with any sorting tab that you select. This affects only the sorting tabs on the non-tag searches: relevance, newest, votes, and active. Also, only "votes" is affected; searching for any of the other tabs does not replicate this effect.
This no longer affects a tag search which begins with the [votes] tag, as indicated by the original report. It appears to have been fixed by proxy when implementing the new tag wiki system. This has drastically reduced the impact that this bug has on users. However, the actual error remains unaddressed, so it still breaks text and compound searches.
As a demonstration, try this search for 'votes:20'. See what happens when you click on the other navigation tabs.
It only affects the first one of these in the search string, so if you did say this compound search for 'votes votes:10', only the first 'votes' would change.
Escaping the word using the " or + literals will avoid this fate - "votes" and +votes both work safely. Thus, I conclude that the cause is ultimately the presence of the first five characters after
q= in the querystring being