I have. I bought a great name from them, after which they voided the purchase and made it a $1,500 premium domain. Obviously, I will never buy domains from Namecheap again.
If we assume that Namecheap is incapable of malice, then it means Namecheap's systems and/or processes are broken. (Registry agreements don't allow registries to retroactively reclassify domains as "premium".)
On what do you base this? Registration agreements allow quite a lot in favor of registrars and registries.