Race Condition?

/**
 * Jquery related stuffs
 */
 function completedBonfire(didCompleteWith, bonfireSolution, thisBonfireHash) {
        $('#complete-bonfire-dialog').modal('show');
        // Only post to server if there is an authenticated user
        if ($('.signup-btn-nav').length < 1) {
            $.ajax({
                type: 'POST',
                data: {
                    bonfireInfo: {
                        completedWith : didCompleteWith,
                        solution: bonfireSolution,
                        bonfireHash: thisBonfireHash
                    }
                },
                url: '/completed-bonfire/'

            });
        }
    }

    $('.next-bonfire-button').on('click', function() {
        var bonfireSolution = myCodeMirror.getValue();
        var thisBonfireHash = passedBonfireHash || null;
        var didCompleteWith = $('#completed-with').val() || null;

        completedBonfire(didCompleteWith, bonfireSolution, thisBonfireHash);

        window.setTimeout(function() {
            // TODO
            window.location = '/bonfires';
        }, 100);

    });
    
    /**
     * The method that handles the post request
     */
  
  app.post('/completed-bonfire/', function (req, res) {
    var isCompletedWith = req.body.bonfireInfo.completedWith || undefined;
    var isCompletedDate =  Math.round(+new Date() / 1000);
    var bonfireHash = req.body.bonfireInfo.bonfireHash;
    var isSolution = req.body.bonfireInfo.solution;
    // TODO
    debug(isCompletedWith, 'Is completed with');

    if (isCompletedWith) {
        var paired = User.find({"profile.username": isCompletedWith}).limit(1);
        paired.exec(function(err, pairedWith) {
            if (err) {
                return err;
            } else {
                var index = req.user.uncompletedBonfires.indexOf(bonfireHash);

                if (index > -1) {
                    req.user.uncompletedBonfires.splice(index,1)
                }
                pairedWith = pairedWith.pop();

                index = pairedWith.uncompletedBonfires.indexOf(bonfireHash);
                if (index > -1) {
                    pairedWith.uncompletedBonfires.splice(index,1)
                }

                pairedWith.completedBonfires.push({
                    _id: bonfireHash,
                    completedWith: req.user._id,
                    completedDate: isCompletedDate,
                    solution: isSolution
                })

                req.user.completedBonfires.push({
                    _id: bonfireHash,
                    completedWith: pairedWith._id,
                    completedDate: isCompletedDate,
                    solution: isSolution
                })

                req.user.save(function(err, user) {
                    pairedWith.save(function(err, paired) {
                        if (err) {
                            throw err;
                        }
                        if (user && paired) {
                            res.redirect('/bonfires');
                        }
                    })
                });

            }
        })
    } else {
        req.user.completedBonfires.push({
            _id: bonfireHash,
            completedWith: null,
            completedDate: isCompletedDate,
            solution: isSolution
        })

        var index = req.user.uncompletedBonfires.indexOf(bonfireHash);

        if (index > -1) {
            req.user.uncompletedBonfires.splice(index,1)
        }
        req.user.save(function(err, user) {
            if (err) {
                throw err;
            }
            if (user) {
                res.redirect('/bonfires');
            }
        });

    }

});

Updated with done

    function completedBonfire(didCompleteWith, bonfireSolution, thisBonfireHash) {
        $('#complete-bonfire-dialog').modal('show');
        // Only post to server if there is an authenticated user
        if ($('.signup-btn-nav').length < 1) {
            $.ajax({
                type: 'POST',
                data: {
                    bonfireInfo: {
                        completedWith : didCompleteWith,
                        solution: bonfireSolution,
                        bonfireHash: thisBonfireHash
                    }
                },
                url: '/completed-bonfire/'

            }).done(function() {
                window.location = '/bonfires';
            })
        }
    }