Short read de novo assembler using de Bruijn graphs, as published in: D.R. Zerbino and E. Birney. 2008. Velvet: algorithms for de novo short read assembly using de Bruijn graphs. Genome Research, 18: 821-829
changed sometime recently. In addition to breaking the SHA1 hash, it also looks like it was packaged incorrectly because it contains a precompiled "velveth" and "velvetg" for Linux x86_64. When building on OS X with "make", those files aren't overwritten, so a "make clean" is necessary before building (probably not what you intended).
The file posted at
http://www.ebi.ac.uk/~zerbino/velvet/velvet_1.2.08.tgz
changed sometime recently. In addition to breaking the SHA1 hash, it also looks like it was packaged incorrectly because it contains a precompiled "velveth" and "velvetg" for Linux x86_64. When building on OS X with "make", those files aren't overwritten, so a "make clean" is necessary before building (probably not what you intended).