Unfortunately, there are times when data is not stored in a GEDCOM properly -- not all the software uses the very latest specification of GEDCOM, and sometimes data can be lost, or garbled. Source notes in particular seem vulnerable.

As with any other data, you should treat information you receive via a GEDCOM as unvalidated until you prove it. It may be tempting to import someone else's research into your family and call it good, but you still have the burden of proof upon you; good research practices demand that you verify all of the information before accepting it.

Some researchers keep multiple databases for their information; one database has information in it that has been proved and sourced, another has information that may be correct, but hasn't yet been sourced. Those researchers always import a GEDCOM into their "unproven" database so they won't get their sourced and unsourced information mixed up.