Challenges with Key with Gaps?

I had a mistake with a trigger which resulted in errors in PostgreSQL 9.4.3 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.9.2-10) 4.9.2, 64-bit.
Each time during the error, the primary key of SERIAL increased.
After fixing the bug, the table measurements is

 measurement_id | measurement_size_in_bytes |             time              
----------------+---------------------------+-------------------------------
              1 |                     77777 | 2015-07-14 18:29:56.858703+03
              2 |                       888 | 2015-07-14 18:29:56.882552+03
              3 |                       888 | 2015-07-14 18:30:15.505957+03
              4 |                       888 | 2015-07-14 18:41:01.878106+03
             39 |                     77777 | 2015-07-15 12:11:21.21391+03
             40 |                     77777 | 2015-07-15 12:11:59.551973+03
             41 |                     77777 | 2015-07-15 12:12:05.48982+03
             42 |                     77777 | 2015-07-15 12:13:02.402053+03
             43 |                     77777 | 2015-07-15 12:13:02.419412+03
             44 |                       888 | 2015-07-15 12:13:02.434728+03
         ...

where the amount of error statements has been 35 (= 39-4).
This jump in IDs may become a challenge later when I want to integrate this table with a partial index where order matters.

Table

CREATE TABLE measurements
    (
        measurement_id SERIAL PRIMARY KEY NOT NULL,
        measurement_size_in_bytes INTEGER NOT NULL,
        time TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
        CONSTRAINT no_duplicate_measurements UNIQUE (time)
    );    

What are the challenges of keys with gaps?