For example, the difference in the syntax for calling indexers between VB and C#
Dim foo As List(Of String) = New List(Of String)
Will not compile with the error - "Error 18 Identifier expected."
If I use the VB syntax in C#, I get my new favorite error: 'foo' is a 'variable' but is used like a 'method'
Particular annoying is the differences in For Each between C#, VB, and Python - each which has a slightly different syntax. For some reason this has been tripping me up a lot during live demos.
Interestingly, the whole curly brace versus END statements versus tabs (python) deal doesn't bother me at all. Although, this shocked me.