Single CPU licences
We're a SAP Reseller of their BI stuff (Crystal & Business Objects). We've had a few customers on old single CPU deals. SAP can't change the licence unless the customer changes version, customers are fairly happy to stick to the old as not enough has changed recently.
Problem is, try finding a single CPU server these days - all multi-core and you pay half CPU for each extra core (rounded up!).
Virtualise we thought - turns out that the old licences didn't specifically include virtualisation rights, therefore you can't virtualise! Would love to see a test case, but SAP have deep pockets.
One customer told SAP to go legal - that led to a pretty decent deal from SAP.
Another have just decided to migrate to a competitor. More of a problem for ERP customers as migration is such a huge task.