The 90/10 rule will apply at the most inopportune moment.
We've all done it. A client has asked how much more work is left on the project and, with excitement, we've exclaimed "I'm 90% of the way there!"
Unfortunately, the final 10% of the project often accounts for 90% of the effort.
Last minute client change requests. Cross-browser/cross-platform testing. Vendor script change integrations. Off-by-1-pixel corrections.
The devil is in the details, and he often lingers alongside the final push to the finish - after we've told the client we're "almost there."
The short lesson: don't ever talk in percentages when you're on the phone with the client. Instead of "we're 90% there," explain that the bulk of the rough work is done and you're now engaged in the final polish of the project. Emphasize the client's role in this process - their feedback (and the clarity of that feedback) is vital to making sure things come to fruition on time.