How do you analyse visual analogue scales with an ambiguous response?
I am not an expert on these scales, but my 2-cents worth would be the following, which are simple work-arounds to include all the data:


Fine as is;
Take the middle of the thick line (the 'average');
Take the center of the circle (draw a cross in it, the intersection is the middle);
Apparently "0" - no pain;
Take the tip as the marker;
Apparently "100".
As in 4. ("0" - no pain);


Points #4 and #6 are obviously the most risky, especially in a small population. In a large population (say N=100) a few outliers don't matter too much. So in case the study sample is small, you may want to consider excluding uncertain cases (#4 and #6 as examples), or try to contact the subjects and re-assess.