In version 12.3.14, MrSID files are drawn at the wrong position. It appears that the size of the error depends on the distance from the map view center.
12.3.13: 
12.3.14: 
12.3.14: wrong image position
Hi Clemens,
This issue has been fixed in the latest version ThinkGeo.Gdal/ThinkGeo.MrSid 12.3.15. Thanks for the reporting!
Ben