Major Description
There are a number of restrictions on valid names in HDFS. For example, you can't name a directory "." or "..", or something containing a ":".
However, I can happily create a snapshot named "a:b:c", resulting in this:
-> % hdfs dfs -createSnapshot /home/andrew a:b:c
Created snapshot /home/andrew/.snapshot/a:b:c
-> % hadoop fs -ls /home/andrew/.snapshot
-ls: java.net.URISyntaxException: Relative path in absolute URI: a:b:c
Usage: hadoop fs [generic options] -ls [-d] [-h] [-R] [<path> ...]