From the included whatsnew.txt file:

1) Faster FFTs for most SSE2-capable CPU architectures.
2) More FFT lengths supported.
3) Some of the new, faster FFTs don't let us do as much error checking on each iteration.
If you don't want to lose this error checking, choose "SUM(INPUTS) error checking" from
the Options menu. This will slow down your tests by about 2-5%.
4) When a possible hardware error occurs during a test, the error message would
likely scroll off the screen unnoticed. Now, all future progress updates to
the screen will be followed by a message containing a count of the errors that
have occurred during the test.
5) Worker Windows now stagger their starts. This improves timings for some
CPUs and operating systems. See undoc.txt to control this feature.

-For rare cases where the program cannot figure out the number of cores and hyperthreading,
the NumPhysicalCores option may help.

1) Occasionally, P-1 stage 2 would report 100% complete before stage 2 completed. This bug has been around forever. I've finally found the cause! Fixed in 26.5 build 2.
2) Worktodo.txt was not updated properly when P-1 completed for a PRP test. Fixed in 26.5 build 3.
3) In 26.5 build 3, when benchmarking and measuring CPU speed the program will keep all cores busy by launching auxiliary threads that loop indefinitely. This should prevent Intel Turbo boost from kicking in. This should prevent cases where the CPU speed is reported as 1.87 GHz, yet a benchmark reports very fast timings because the CPU core has been boosted to 3.2 GHz because only one core is active.
4) In 26.5 build 3, the OS's mapping of hyperthreaded logical CPU numbers to physical CPUs is determined automatically at startup. The AffinityScramble setting (see undoc.txt) is no longer supported. It is replaced by the AffinityScramble2 setting.
5) In 26.5 build 3, the message "set affinity to run on any cpu" instead listed all the CPU numbers and as well as CPU #31. Fixed in build 4.
6) In 26.5 build 3 and 4, benchmarking a dual-core non-SSE2 machine will crash. Fixed in build 5.
7) KeepPminus1SaveFiles=0 option described in undoc.txt did not work when a factor was found. Fixed in 26.6.
8) The 32K,64K,80K length FFTs with sumout checking for Pentium 4's with 256K or less cache were not implemented. Symptom is a torture test failure. Fixed in 26.6.
9) Prime95 does not load on Sandy Bridge CPUs running Win XP. I've got a trial fix in 26.6.
10) Prime95 chooses very inefficient FFT implementations for Core 2 Celerons (they have 512K L2 cache). Working on a fix for for 26.6.
11) Logical CPU numbers were sometimes output zero-based and sometimes one-based. In 26.6, they are all one-based.