Never seen this happen on proper (even if it is really messy

) code
Possibly (not really a bug. GIGO

)
Lousy (malformed?) XHTML code.

validate before split/global-S&R. Validate AGAIN after a split/global-S&R
BEFORE SAVING
(java) Scripted Code in the body. Can confuse things
NOW Using Sigil 0.8.6 (W7, XP) but have used every version in the past (except .1

)