I was checking current trunk (r66532), it acts strangely:

1. :
   1. Create a LLD rule, go to Filters tab, add one filter
   2. Choose Type of calculation: And
   3. Press Update, no errors
   4. View LLD rule, note that And was saved
2. :
   1. Create a LLD rule, go to Filters tab, add one filter.
   2. Choose Type of calculation: Or
   3. Press Update, no errors
   4. View LLD rule, note that Or was saved
3. :
   1. Create a LLD rule, go to Filters tab, add one filter.
   2. Choose Type of calculation: Custom
   3. Enter a valid expression, e.g. "A"
   4. Press Update, no errors
   5. View LLD rule, note that frontend silently saved And/Or
4. :
   1. Create a LLD rule, go to Filters tab, add one filter.
   2. Choose Type of calculation: Custom
   3. Enter an invalid expression, e.g. "not A"
   4. Press Update, no errors
   5. View LLD rule, note that frontend silently saved And/Or
5. :
   1. Create a LLD rule, go to Filters tab
   2. Add one filter
   3. Add a second one
   4. Remove the second filter
   5. Note that Type of calculation drop down disappears!