Spinoff from this discussion:
Right now CheckIndex claims the index is corrupt if you index a Token with -1 position, which happens if your first token has positionIncrementGap set to 0.
But, as far as I can tell, Lucene doesn't "mind" when this happens.
So I plan to fix CheckIndex to allow this case. I'll backport to 2.3.2 as well.
LUCENE-1253 is one example where Lucene's core analyzers could do this.