Water is probably the number one cause of false positive AFB stains.  We had 
a horrible problem with it many years ago.  Acid-fast organisms have been 
reported growing (well present at least) in 40 gal formalin tanks, in the old 
Tissue Mat paraffin wafers and in a tremendous number (over 30 %) of water 
fountains.  We even found AFB in our gelatin that we used in the water bath. 
This is all information from published papers.  While many of the organisms 
may not be pathogens, they are acid fast.  We finally went to Millipore 
filtered water, that had already been through a charcoal filter and 
deionizing column, for water baths and for all water changes before the 
carbol fuchsin stain.  We did use staining jars but filter the reagent each 
time.  I strongly recommend a negative control cut from a case processed the 
same day as the requested AFB tissue.  We always picked up extra sections 
from a hysterectomy case and ran them along with the stain.  If there is any 
doubt about contamination, the negative controls are very, very carefully 
examined.  Using this type of negative control (from the same day's workload) 
checks for contamination at every step of the procedure.

Freida Carson