"Someone screwed up big time," one source maintains in estimating to GlobeSt.com that the faux pas could amount to "a $100-million mistake" given the length and terms of the lease, which reportedly escalates to a near-record rate for Cambridge of $72 per sf. When signed earlier this summer, the agreement was hailed as a landmark deal for the building and the market. Microsoft was expected to move close to 1,000 employees to One Memorial Dr. during a three-part lease up slated to begin next month.

When contacted by GlobeSt.com, EOP had no comment beyond stressing that work is proceeding on the first piece of Microsoft's relocation, while an official at PA Consulting says she is unaware of any dispute. A Microsoft spokesman declined to say whether its lease would be impacted, only offering that, "since this is a private dispute to which we are not a party, we have no comment."

The overlooked tenant reportedly has an option for space on the 15th, 16th and 17th floors. The problem, according to sources, occurred because the option was not addressed when Microsoft agreed to expand its commitment to One Memorial Dr. from an initial lease signed in April. Under terms of the final lease, Microsoft was to begin occupying the top three floors by 2009 while moving to the first, second, ninth, 10th and 11th floors in the two initial rounds.

In one scenario offered by brokers, Microsoft could argue that Blackstone is in default of the lease for being unable to deliver the space as promised. Others speculated that EOP might try to craft a settlement with PA Consulting that would allow the Microsoft lease to remain in place. Blackstone/EOP owns substantial space in Boston, including some of its top office towers.

Observers spoken with also debated the frequency of missing such an option. One industry veteran claims it is not particularly uncommon, but a colleague calls the transgression "very rare."