The timeline images are either generated without text (only graphs, no legends, no descriptions...) or the image is not generated at all.
Take first example timeline on http://www.mediawiki.org/wiki/Extension:EasyTimeline
original:
http://upload.wikimedia.org/wikipedia/mediawiki/timeline/d12a83ed39e720e3d4eb41241afdef3d.png
slight change [shift:($dx,-12) to shift:($dx,-11)] - no text:
http://upload.wikimedia.org/wikipedia/mediawiki/timeline/785bb748606c385fec46d0d16b032390.png
another slight change [some random playing with numbers] - no image at all
http://upload.wikimedia.org/wikipedia/mediawiki/timeline/c074568e9acc4b0d6f652a2ac2fbd648.png
Sometimes besides nonexistent image HTML source also contains only empty <map> tag without any <area> tags.
Version: unspecified
Severity: major