When a question is migrated from MO to MSE, the system attempts to retag it using MSE tags (taking into account synonyms on the MSE side only; synonyms are a one-way street). If no MSE tag can be found, then either
- migration fails, if it was user-initiated; or
- migration goes through with the question tagged untagged, if it was moderator-initiated.
This was discussed on meta.MO, where Willie Wong suggested
every time a migration fails for this reason, flag down Mariano and ask him to create the appropriate synonym at MSE.
I do not think this solves the problem. For one thing, a MO user whose close vote is decisive is likely to be voting from a review queue, and will not be made aware of the outcome of the vote. Other users, should they come across the question, will see it closed as off-topic, without evidence that it was intended for migration. Looking at the MSE tag synonyms, I suspect that Mariano is not getting a lot of requests of this kind.
Another, and bigger concern for me is that sometimes migration goes through due to some inessential tag being in common, while the essential tag is lost. For example, a question tagged with at.algebraic-topology reference-request will end up with only reference-request on the MSE side. One question with gt.geometric-topology surfaces ended up with surfaces. Having such a tag as the only tag is worse for the question than being untagged; it's a one-way ticket into Tumbleweed land.
I went through last month's migrations from MO to MSE, and found that the following tags were lost in transition, although they could easily be mapped to MSE analogues.
ag.algebraic-geometry -> algebraic-geometry at.algebraic-topology -> algebraic-topology co.combinatorics -> combinatorics fa.functional-analysis -> functional-analysis gr.group-theory -> group-theory gt.geometric-topology -> geometric-topology lo.logic -> logic na.numerical-analysis -> numerical-analysis oc.optimization-control -> optimization pr.probability -> probability eigenvector -> eigenvalues-eigenvectors eigenvalue -> eigenvalues-eigenvectors group-rings -> group-ring limits -> limit linear-optimization -> linear-programming linear-pde -> pde matrix-analysis -> matrices recurrences -> recurrence-relations
Should the above be made tag synonyms?
I emphasize that these are actual examples from migrated questions during the last month only. There are other common prefixed MO tags which did not have a question migrated during this time span.
Added: I'd like to quote Noah Snyder's prophetic words posted on July 21, 2010, the second day of MSE private beta.
it might be convenient to have some similarity in tagging between MO and here so that if and when questions can be migrated directly between them we won't have to do lots of retagging