For some time I have been wondering if the anecdotal evidence about foods containing GMO crops (my cat won't eat it, I seem to be allergic to it, etc.) isn't more about glyphosate residues than about the effects of genetic engineering. The EPA is re-examining glyphosate for evidence of carcinogenic properties, but no one really knows what long term exposure (its use was minimal until the last few decades) to low levels is doing. It may be a hormone disrupter or contributing to increased antibiotic existence.
Glyphosate residues are now found in 38 states, in many waterways, in rain water, and in the systems of farm workers. Some countries are banning it or considering a ban.
Read a nice review of the current state of our knowledge here: