Most CRM systems have a lead source field on the lead, contact, and even opportunity objects. These are fine for toy systems, and they're immediately satisfying to the novice user. But they are an abomination because they can't represent the multiple "touches" that are the reality of sales cycles. Furthermore, in too many systems, users are allowed to change the lead source at will. The right path: use campaigns and activities that show the chronology of marketing actions and prospect responses.
Number 5: Don't Enter Any Information in the CRM Until Just Before the Deal Closes
A classic behavior stimulated by inappropriate sales management is sales reps not entering deals until the very last minute, to register their win with zero risk of losing. In itself, this doesn't corrupt data...but it does commit a deliberate sin of omission: you can't tell how long the sales cycle really takes, and you can't tell what your win-rate is. The corruption here isn't data, it's business process and management.
Number 4: Change the Amount Field When You Lose an Opp
Another classic of sales mismanagement is to allow reps to change opportunity amount values to $0 when a deal is lost. This is a great way to make it look like the only deals you lose are the worthless ones. A variation of this folly is to change the value of the opportunity amount field to the overall size of the prospect revenues. This is a great way of indicating the size of the target market, but it's an even better way of corrupting the meaning of the pipeline.
Number 3: Don't Lock Down Fields
Once things have started to percolate with a lead, contact, or opportunity, it's important to lock down the values of key fields: lead source, amount, close date, opportunity owner, etc. To not do this is to invite silly misrepresentations of your sales cycle.
Number 2: Let Users Delete Anything They Want
If fields must be locked down, the ability to delete entire records should be carefully regulated. Users tend to hide perceived weakness, so they'll delete records. Instead, these records should be zapped so they disappear from view, but can still be analyzed for process troubleshooting. Extra--Extra--Read all about it here.
And the Number 1 way for users to corrupt CRM data...
Let Users Write All their Own Reports