... [Tim Peters]
Conflict errors are certainly normal, but IIUC the OP is seeing site errors, not just log msgs ...
[Chris McDonough]
This is true. I misunderstood the OP inasmuch as I thought he was seeing the error message in his event log, which would be normal It is certainly the intent that end users should never see this error message in a browser, but it's not a promise as it depends how the developer's code is written. Zope will retry a request three times due to conflict errors before it gives up and presents the conflict-caused error to the user.
So maybe it's really a Plone question (as in, is Plone doing something here for member registration that makes read conflicts unusually likely?); or maybe the OP's box is overburdened, so that transactions take much longer than "normal" (e.g., too little RAM, whatever); or ... It would at least be interesting to know how often conflict errors are just _logged_ (as opposed to surviving long enough to show up to the end user).