The code in the Trailhead Project called "Creating Apex Triggers" is incorrect in lines 4 and 10. 
Here is the existing code:
03     //collect ID's to reduce data calls
04     List<Id> speakerIds = new List();  
05     Map<Id,DateTime> requested_bookings = new Map<Id,DateTime>();
06     
07     //get all speakers related to the trigger
08     //set booking map with ids to fill later
09     for(Session_Speaker__c newItem : trigger.new) {
10         requested_bookings.put(newItem.Session__c,null); /
11         speakerIds.add(newItem.Speaker__c);

Here is the revised code:
03     //collect ID's to reduce data calls
04     List<Id> speakerIds = new List<Id>();  
05     Map<Id,DateTime> requested_bookings = new Map<Id,DateTime>();
06     
07     //get all speakers related to the trigger
08     //set booking map with ids to fill later
09     for(Session_Speaker__c newItem : trigger.new) {
10         requested_bookings.put(newItem.Session__c,null);
11         speakerIds.add(newItem.Speaker__c);

Notice there is no / at the end of line 10.  Thanks to my super-dev guy Kyle for helping me with this!

The code in the Trailhead Project called "Creating Apex Triggers" is incorrect in lines 4 and 10. 
Here is the existing code:
03     //collect ID's to reduce data calls
04     List<Id> speakerIds = new List();  
05     Map<Id,DateTime> requested_bookings = new Map<Id,DateTime>();
06     
07     //get all speakers related to the trigger
08     //set booking map with ids to fill later
09     for(Session_Speaker__c newItem : trigger.new) {
10         requested_bookings.put(newItem.Session__c,null); /
11         speakerIds.add(newItem.Speaker__c);

Here is the revised code:
03     //collect ID's to reduce data calls
04     List<Id> speakerIds = new List<Id>();  
05     Map<Id,DateTime> requested_bookings = new Map<Id,DateTime>();
06     
07     //get all speakers related to the trigger
08     //set booking map with ids to fill later
09     for(Session_Speaker__c newItem : trigger.new) {
10         requested_bookings.put(newItem.Session__c,null);
11         speakerIds.add(newItem.Speaker__c);

Notice there is no / at the end of line 10.  Thanks to my super-dev guy Kyle for helping me with this!