For legal issues with very little case law, courts may decide it is important to not disrupt settled expectations.
(Programmers may liken this to "the principle of least surprise".)
 Whose expectations?
An absurd argument that might be made is that the court should avoid surprising patent lawyers. Given that patent lawyers neither make nor sell software, this is simply ludicrous and shouldn't be difficult to argue against.
The court should take more interest in avoiding surprising software developers, distributors, businesses, and users.
This principle has been used by the US Supreme Court:
[...] the guidance of Warner-Jenkinson [Supreme Court decision], which instructed that courts must be cautious before adopting changes that disrupt the settled expectations of the inventing community.
Source: Festo, 535 U.S. at 739 (28 May 2002)
(Need to check: Was the Supreme Court talking about the expectations of patent applicants regards what the Supreme court says and will say, or are they talking about the broader sense of what patent applicants expect from the PTO and the courts in general?)
The above was quoted by the US CAFC in an en banc ruling:
In addition to the statutory language and Supreme Court precedent supporting the existence of a written description requirement separate from enablement, stare decisis impels us to uphold it now. Ariad acknowledges that this has been the law for over forty years, see Appellee Br. 24, and to change course now would disrupt the settled expectations of the inventing community, which has relied on it in drafting and prosecuting patents, concluding licensing agreements, and rendering validity and infringement opinions. As the Supreme Court stated in admonishing this court, we “must be cautious before adopting changes that disrupt the settled expectations of the inventing community.” Festo, 535 U.S. at 739; see also Watson v. United States, 552 U.S. 74, 82 (2007) (“A difference of opinion within the Court . . . does not keep the door open for another try at statutory construction, where stare decisis has special force [since] the legislative power is implicated, and Congress remains free to alter what we have done.” (internal quotations omitted)). If the law of written description is to be changed, contrary to sound policy and the uniform holdings of this court, the settled expectations of the inventing and investing communities, and PTO practice, such a decision would require good reason and would rest with Congress.
Case: Ariad Pharmaceuticals, Inc. v. Eli Lilly and Co., 2008-1248 (Fed. Cir. Mar. 22, 2010)
Similar can be seen in Symbian v. Comptroller General (2008, UK), where the judge concluded that many interpretations of the EPC were possible, and decided to follow the EPO's interpretation simply to avoid diverging with the Board
51. These considerations reinforce our view that, at least in this court at this stage, we should try to follow previous authority, we should seek to steer a relatively unadventurous and uncontroversial course, and we should be particularly concerned to minimise complexity and uncertainty. These aims are not necessarily mutually consistent, but, on this occasion, we believe they are achievable, namely by following the analysis adopted by the Board in Vicom and the two IBM Corp. cases, and of the Court of Appeal in Merrill Lynch and Gale.
58. Indeed, it appears to us that upholding the conclusion of the Comptroller in this case, would involve the English courts departing from all the decisions of the Board to which we have referred. ... 
The "Board" is the EPO Technical Board of Appeal, which is lower down in the hierarchy and is not even controlled by the UK (nor is it controlled by the EU for that matter). There is no legal or institutional reason for the UK Supreme Court of Judicature to follow the Board's decisions. They seem to have done so to avoid disrupting settled expectations - but they've made the mistake of thinking that the expectations of patent lawyers are more important to software developement than the expectations of software developers.
This is also visible in the ruling given when the case was heard on appeal by the High Court of Justice (just below the UK Supreme Court):
This divergence between the jurisprudence of the English Courts and the EPO is a matter of considerable concern and renders hollow the sentiments expressed by judges such as Lord Hoffmann in Merrill Dow Pharmaceuticals Inc. v H.N. Norton & Co. Ltd  RPC at p.82 about the need for consistency of approach.
It's surprising, and worrying, to see the UK courts taking direction from the EPO.
 Implications for brief drafters
We have to ensure that the court knows that "the inventing community" for software is software developers, not patent lawyers. In other fields, patent lawyers and product developers are two parts of the same entity (a company), so the court might be right in accepting the patent lawyers' opinion as the legal expression of the opinion of the developers. In software, this isn't the case.
 Related pages on en.swpat.org
- Ariad v. Lily: Choosing to Not Disrupt the Settled Expectations of the Patent Community, 28 Mar 2010, Patently-O
- Disrupting settled expectations, 5 Apr 2010, 717 Madison Place
- A 2012 amicus brief from the pro-swpat BSA, using settled expectation to argue for a pro-software-patent ruling in CLS Bank v. Alice (2012, USA)
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