rack/rack — comment on commit 4c58e4d3088c08ae54f8c1883b62f6e727ea45d4
File: lib/rack/backports/uri/common_18.rb line 12
By stefanoc:

Under 1.9.3-p125, by the time this line is reached the hash has already been frozen (twice, actually) by STDLIB's uri/common.rb (line 856). This results in a "can't modify frozen Hash" error during Rails bootstrapping, for instance. The same app boots fine under 1.9.3-p0.