Caveat emptor.

"If the reagents used for building libraries themselves (or those used for amplifying and confirming the clones following screening experiments) inadvertently contain cross-contaminated oligos, it will make the situation more complex.
We realized this possibility recently, as we observed some unexpected results (presence of unrelated sequences) when setting up a CRISPR library screening, which prompted us to investigate the issue further. Our systematic analyses of various reagents, research equipment, and multiple vendor-supplied oligos suggested that commercially synthesized oligos are the source for such cross-contaminants."

https://academic.oup.com/nar/advance-article/doi/10.1093/nar/gkae068/7602849?login=false