It appears bulk export (or Publish) of images puts entries into the Undo stack for each image. If the user carries on developing other images whilst the bulk export is progressing then the user cannot undo such things as slider changes because the Undo stack is full of recorded export state changes. At worst if the user performs a sync develop change across a large number of images during an export then they may be put in a position where they cannot undo such a change potentially leading to alot of manual work.
The issue has been discussed here in the context of LR 3 but after testing it in LR4 I raise it for consideration … http://forums.adobe.com/message/4222032#4222032
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