i have had experiences with both. I had a chemical pregnancy last month. I was getting light positives that were progressively getting darker and then AF came and then the tests went negative.
Chances are, what you seen was an evap line. Evap lines come after the time limits and they are very common on cheapo tests atleast for me.. not everyone gets them i guess a lot depends on what kind of test. For me, i used to do well on ebay tests and now all i get is evap lines every time and they were convincingly positive but i wasn't pregnant. Last month my lines were on Answer and they came up right away so i know they weren't evap lines.
Evap generally doesn't have color.. its usually a shadow or gray.. though the ebay tests i had, some were even pink lines. i guess its always possible it was another chemical preg but i guess i wont ever know..
Hope that helped

if not, feel free to email me and i can explain more
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Formerly~Stressedmama/Amanda81~
Amanda-Happily married to James (2/01)
Mom to Sarah 5/98, Mikayla 5/01, Tyler 10/02 and Noah 10/03
and expecting a surprise August 2005.